Original air date: April 6, 1967

In the penultimate episode of the first season (credited to Harlan Ellison^note who wrote the story but not the teleplay - see the Trivia tab for all the gory details), the crew of the Enterprise finds itself exploring a "temporal disturbance" near an unknown planet. Electromagnetic interference causes the ship's computer to explode, injuring Sulu. McCoy runs to prepare an injection of super-adrenaline for him, but turbulence causes him to accidentally inject himself and become Brainwashed and Crazy. McCoy escapes the bridge and beams down to the planet.

Following him, the landing party discovers a city of ancient ruins that appears to be uninhabited and empty except for a mysterious glowing green archway, which appears to be sentient and exhibits strange telepathic abilities, acts as a viewscreen displaying various scenes from Earth history. The archway, which calls itself the Guardian of Forever, explains that it is a portal to any place and period in history. It also easily steals Spock's Cultural Posturing crown.

The conversation with the Guardian is interrupted when McCoy runs through the archway and disappears. Moments later, Uhura reports a loss of contact with the Enterprise, and the crew realize that McCoy has somehow altered the course of history since entering the portal. In order to return to their ship and restore the original timeline, they must follow him and prevent him from doing whatever he did. Steeling themselves to find the correct moment in history, Kirk and Spock jump through the portal and arrive in New York City during The Great Depression, where the pair must disguise themselves while they search for McCoy. Unfortunately, their Starfleet uniforms (and Spock's Vulcan ears) attract unwanted attention. When the pair attempt to steal clothes, they are chased by the police and end up hiding in the basement of a homeless shelter run by the kind, idealistic and smart social worker Edith Keeler (Joan Collins). Keeler offers to take in Kirk and Spock and find them work, although she is slightly suspicious of their military demeanour.

While Kirk and Spock do menial jobs, Spock is able to construct a rudimentary computer using electronic parts he has assembled, which he uses to analyze his tricorder's recording of the historical scenes shown in the archway. Kirk and Edith begin to fall in love. Spock discovers a terrible truth: Soon after he entered the portal and arrived on Earth, McCoy saved Keeler from dying in an auto accident. Since she did not die as intended, she later became a peace activist and successfully convinced Franklin D. Roosevelt to keep America from entering World War II until too late, allowing the Nazis to win and preventing Starfleet from ever existing. Kirk and Spock realize that in order to prevent this from happening, Edith Keeler must die.

Meanwhile, McCoy has been running loose in the city, still affected by the drugs. After an altercation with a homeless man who steals his phaser and shoots himself with it, he is found and taken in by Keeler, who believes he is merely drunk. As he recovers from the drug's effect, he befriends her, unbeknownst to Kirk and Spock.

The following night — the night of her accident — Kirk takes Edith on a date to the movies. An offhand comment by Edith reveals to Kirk that she knows McCoy. When Kirk and Spock run into McCoy on the street, Edith walks into traffic to find out what's going on. Kirk instinctively moves to push her out of the way, but Spock insists he not interfere. When McCoy moves to save her, Kirk restrains him, and Edith is struck by a car and dies, returning history to its original course.

McCoy is shocked and furious, demanding to know why Kirk stopped him from saving Edith and if he knows what he's done. Kirk is too heartbroken to say a word, and Spock gently tells Bones that "he knows". With the timeline restored, the three are able to return to the planet through the portal, where the Enterprise has reappeared. Scotty expresses surprise at the short time that's elapsed since they left. The Guardian offers to show the crew more fantastic journeys, but an emotionally shaken Kirk simply states "Let's get the hell out of here" and prepares to have the crew beamed back aboard.

In 2015, IDW published a comic book adaptation of Harlan Ellison's original script of this story.

The Tropes on the Edge of Forever

• Accidental Suicide: A vagrant picks up Bones' dropped phaser and disintegrates himself while handling it.
• Alternate-History Nazi Victory: A delusional Bones is sent back in time and saves the life of Edith Keeler, who then goes on to convince President Roosevelt to stay out of the war. This is a downplayed example since the consequences are never shown beyond the Enterprise being erased from existence. Nazi Germany's victory meant that mankind continued to be involved in petty internecine squabbles and never became a space-faring civilization.
• Always Save the Girl: Averted. A doped up McCoy destroys the Federation by saving Edith Keeler. Kirk and Spock go back to find McCoy and realize that she is the very one that MUST die for the timeline to be restored. Ultimately, Kirk decides he must let her die, but it's a difficult decision.
• America Won World War II: Implied, since it's stated that America staying out of the war in Europe apparently allows Nazi Germany to defeat both Britain and Russia. Somewhat justified as the Nazis develop nuclear weapons as a result and that's not good for anybody.
• Anachronism Stew: The story is set in 1930, yet "Goodnight, Sweetheart" was recorded in 1931.
• Artistic License – History:
  • Edith's fate is pinned on the reveal that neither Kirk nor Bones know who Clark Gable is. The episode takes place in 1930, so in fact Edith shouldn't have known who he was, either - he was just a bit player in 1930; he didn't become a star for another two years. Reportedly at one point the script used the name of a lesser-known (to 1960s audiences) actor named Richard Dix, but by the time the episode was filmed it was decided to use the better-known Gable, despite the anachronism.
  • It's insanely unlikely that one outspoken pacifist could have single-handedly sabotaged the Allied cause so significantly. Regardless of her motives, Edith would have almost certainly been seen as just another isolationist like Lindbergh. After all, the America First Committee did attract its share of genuine pacifists. And how exactly is she supposed to have succeeded where Lindbergh failed? We know she gets popular in the alternate timeline, but does her fame really eclipse that of Lucky Lindy?
  • Changing history by making the USA enter the war late or not enter it at all would have many, many consequences which could possibly help the Nazis win World War II (and it should be noted that "Hitler wins the war" does not necessarily mean "Hitler conquers the world," though even a negotiated Nazi victory would undoubtedly still be a very unpleasant geopolitical condition). Britain could be deprived of vital supplies and eventually forced to surrender. Russia could be unable to launch their great 1942-1943 counteroffensive without all those means of transport and communication they got from the Americans. Japan could build its empire in Asia unopposed. However, the screenwriters decided to pick one factor on which the USA not partaking in war would have little to no impact whatsoever. Contrary to popular belief, the Nazi nuclear program was not nearly as advanced as the Manhattan Project, and it was successfully derailed in 1943 by the British Special Operations Executive, without any American involvement.
  • Spock claims that with V2 ballistic rockets at their disposal, Nazis would be able to deliver nuclear payload anywhere they want. In reality, V2's maximum range was only 380 kilometers — not even close enough to endanger the USA or other distant allied powers. In addition, the V2 rocket had a payload that weighed about 2,000 pounds. That was pretty much the limit that it could carry, so it never would have been able to even get off the launch pad with a primitive nuclear weapon based on 1940's technology. Fat Man and Little Boy, the atomic bombs dropped on Japan, each weighed just over 10,000 pounds. The Germans did have some more advanced delivery systems, such as the long-range Amerikabomber project, the V5 multistage ballistic missile, and the Silbervogel orbital bomber (whose concept was eventually developed into the Space Shuttle), but only the first of these ever made it into the prototype phase.
• The Bad Guy Wins: In the alternate timeline, Adolf Hitler conquers the world. And as if the stakes needed to be any higher, a side effect of Enterprise's Xindi arc is that in this timeline there is also no Jonathan Archer to stop the Sphere Builders.
• Bamboo Technology: Spock has to create a massive crude circuit array for his tricorder to display its recordings from the Guardian, and he complains that he has to work with the 1930s equivalent tech resources of "stone knives and bear skins." Of course, when Kirk dryly remarks that it would be an extremely complex problem in logic, and that perhaps he expects too much of Spock sometimes, the science officer is quick to take up the challenge.
• Been There, Shaped History: Kirk, Spock, and Bones were already there in 1930. If they had not have been, Edith would not have been distractedly crossing the street at that exact moment... and would have not been struck by the truck.
• Big Applesauce: Of all the places on Earth to which Bones (and later Kirk and Spock) could travel, he happens to wind up in New York City.
• Bittersweet Ending: Edith dies, leaving Kirk a broken man, but the future continues as normal.
• Blatant Lies: Kirk tries to pass off Spock's ear shape as being the result of surgery after an accident with a mechanical rice picker while in Asia. Naturally, no one buys it. Spock's incredulous reaction to such a feeble lie is priceless.
• Born in the Wrong Century: Edith's ideals of peace came off as preachy and unattainable in her era, but were accepted in Kirk's future era, which is what draws him to her.
• Butterfly of Doom: Spock explains to Kirk that because Edith Keeler was not hit by a car and killed in 1930, she became the leader of a pacifist movement that delayed American entry into World War II for so long that the Nazis developed the atomic bomb first, and neither Starfleet nor the Enterprise ever existed.
• Clothesline Stealing: Kirk and Spock take clothes to blend in in 1930s America. They get noticed by a police officer.
• Cool Gate: The Guardian of Forever is a sentient stone ring that can send people pretty much anywhere and anywhen.
• Cover Version: The 1931 popular song "Goodnight, Sweetheart" (written by Ray Noble, Jimmy Campbell, and Reg Connelly) is performed by the series orchestra (in an arrangement by the episode's composer, Fred Steiner), though it is shown to be playing on the radio in the scene (marking a rare use of diegetic music for the series). Accordingly the arrangement is very much in the style of popular music in the 1930s, with a brief vocal section (provided by an unknown and uncredited session vocalist) followed by a lengthy instrumental section.
• Cultural Posturing: For once, Spock ends up on the receiving end of a conversation about how superior and advanced the speaker's culture is compared to the listener's. After Spock admits to Kirk that the mysterious archway which has been causing the time disturbances operates by no science he understands, the archway itself enters the conversation:
  Kirk: Then what is it?
  Guardian: A question. Since before your sun burned hot in space and before your race was born, I have awaited a question.
  Kirk: What are you?
  Guardian: I am the Guardian of Forever.
  Kirk: Are you machine, or being?
  Guardian: I am both... and neither. I am my own beginning, my own ending.
  Spock: I see no reason for answers to be couched in riddles.
  Guardian: I answer as simply as your level of understanding makes possible.
  Spock: A time portal, Captain. A gateway to other times and dimensions, if I'm correct.
  Guardian: As correct as possible for you. Your science knowledge is obviously primitive.
  Spock: (Fascinating Eyebrow) Really.
  Kirk: (enjoying every minute of this) Annoyed, Spock?
• Cut Himself Shaving: Kirk stumbles for an response when a suspicious cop notices Spock's pointed ears. Finally, after Spock suggests "the unfortunate accident I had as a child", he explains that Spock caught his head in a mechanical rice-picker.
• Deadly Hug: A very unusual variant. Kirk kills Edith Keeler by hugging not her but McCoy, thus stopping McCoy from pushing Keeler out of the way of a car. It's poignant because Kirk loves Keeler, but she was going to unintentionally destroy the Enterprise, not to mention plunge the world into a Nazi dark age.
• Disposable Vagrant: Sure hope that bum who accidentally phasered himself didn't have any important descendants...
• Double-Meaning Title: The title refers to both the dead city on the time planet and New York itself, where the timeline will either be restored or disrupted. In the original script, Kirk, upon first seeing the city sparkling like a jewel on a high mountaintop, reverently says it looks like "a city on the edge of forever". In the first treatment for this episode, the city they travelled back in time to was Chicago.
• Downer Ending: Kirk goes after McCoy after the latter briefly goes mad on an experimental drug. The two end up traveling back in time, where Kirk meets Edith Keeler. They hit it off and it seems like he's finally found true love, but it's then revealed that she's doomed to die in a traffic accident. Kirk thinks about saving her life- only to learn that if she survives, she'll start a pacifist group that will delay the United States's entry into World War II, which will in turn allow Hitler to develop atomic technology and defeat the Allied Powers. Despite truly loving the woman, Kirk is forced to watch, knowing full well what's about to happen, as she starts crossing the street where she'll be struck. This episode ends with the only profanity uttered in the original series: Kirk's famous line "Let's get the hell out of here."
• Dramatic Irony: The very ideals and beliefs which Edith Keeler espouses (achieving world peace, traveling into space, curing disease, feeding the hungry) are all ones which are achieved in the time period of the series under Starfleet. But in order for this to happen, not only must the United States enter a great world war first that will kill millions of people, Edith herself must die.
• Dramatic Shattering: A random vagrant is so frightened by Bones' appearance that he drops a glass bottle of milk.
• Explosive Instrumentation: At the beginning of the episode, leading to Bones breaking out the cordrazine.
• Faeries Don't Believe in Humans, Either: McCoy's response to Edith Keeler's understandably skeptical reaction to his drug-induced delirious mumblings about who he is: "That's all right. I don't believe in you either." (In fact, he initially dismisses his whole situation as some kind of Cordrazine-induced hallucination.)
• Fish out of Temporal Water: When they first arrive in New York in 1930, Kirk and Spock stand out like sore thumbs because of their Starfleet uniforms and Spock's Vulcan ears. Even after they are able to "borrow" clothes (and a woolly hat to hide Spock's ears), their ideals and behaviour mark them as strangers to the era, which does not escape Edith's notice.
• Foreshadowing:
  • Almost the first thing to happen to Kirk and Spock when they arrive in the past is that they're nearly run over by a car.
  • After Spock first finds the newspaper footage of both Edith's obituary and her apparent future meeting with President Roosevelt, he points out to Kirk that they don't yet know whether McCoy will kill Edith, or prevent her death. The next scene has McCoy appearing in the street and chasing a terrified man while shouting "I won't kill you! It's they who do the killing!" It also cuts right to Kirk when McCoy screams "murderers!" for the first time.
• Gaussian Girl: Edith Keeler is exceptionally blurry in all close-ups, even by Star Trek standards.
• The Glomp: Bones gets a bearhug from Kirk near the end when the latter and Spock are overjoyed to finally find him, but the happy celebrations are cut short because Edith Keeler is hit by a car seconds later.
• Godwin's Law of Time Travel: To quote Spock in David Morgan-Mar's treatment of this episode in Planet of Hats, "It's the First Law of Time Travel. [...] Time travel: ergo, Hitler." Bones going back to 1930 and preventing Edith Keeler's death causes the Nazis to win World War II.
• Gone Horribly Wrong: In an alternate timeline, the survival of Edith Keeler led to an influential peace movement which later delayed US entry into WWII, resulting in Adolf Hitler winning the war and the Space Age never happening. Everyone else in the galaxy is presumably screwed, too, as there is now no Jonathan Archer to stop the Sphere Builders.
• Good Samaritan: Edith Keeler embodies this trope perfectly. She runs a soup kitchen for the homeless and downtrodden in Depression-era New York and truly believes in the inherent goodness of man. So of course, she's fated to die.
• Hard Truth Aesop: "Pacifism is an ideal to aspire to, but reality is more cynical, and sometimes people must be prepared to fight." It's not necessarily a bad aesop, but it's certainly more cynical than you'd expect of a 60s show, and in rather stark contrast to the pretty strong "Pacifism = Good" message sent by earlier episodes. It can get worse when you consider the time the episode aired, and see it as a possible call for American intervention in Southeast Asia.
• Heroic BSoD: Kirk ends the episode in a stunned, angry state after Edith Keeler's death.
• Hide Your Otherness: Spock covers up his very noticeable pointed ears in the 1930s by wearing a wool cap.
• I Did What I Had to Do: James Blish wrote the short-story adaptations for many of the episodes. When he wrote this one, it's subtly hinted that Spock was going to kill Edith himself, if that was what it took to restore the timeline. In the actual episode, Spock does -very gently- chide Kirk for catching Edith when she was falling down a flight of stairs, reminding him that he will have to think, not act, when the moment comes.
• Informed Attribute: For a Guardian of Forever, it certainly doesn't do anything to stop a drug-addled crazy man from running into the past and screwing with human history, the sort of thing a Guardian would be expected to prevent. Maybe it would have been more accurate to call it a 'Watcher' seeing as it's a gateway. Its duties don't seem to include making changes to the timeline itself.
• Irony: Kirk and Spock, to save the Federation, have to deliberately allow Edith Keeler to die while the men who destroy the Federation will accidentally do so by saving Keeler. The original treatment by Ellison even has Kirk and Spock talking about how the Villain of the Week, who is as contrary to the standards of the Federation as they go, saved Keeler in a rare moment of humanity.
• Kill the Cutie: Edith Keeler devotes her life to helping the many poor and hungry of the Great Depression, and is convinced that humanity can achieve great things if they could only set aside their petty squabbles. Kirk is fascinated by her altruism and idealism and falls in love with her. And then Spock tells him that she must die if the timeline is to be restored.
• Large Ham: Drugged-up McCoy gives the normally restrained DeForest Kelley an opportunity to chew the scenery as McCoy screams about killers and assassins.
  McCoy: KILLERS! ASSASSINS! I won't let you...! I'LL KILL YOU FIRST!
• Leitmotif: "Goodnight, Sweetheart" is Kirk and Edith Keeler's song.
• Look Both Ways: Edith Keeler is hit by a car as she crosses the street, while Kirk prevents McCoy from saving her in order to restore the original timeline.
• Mad Scientist Laboratory: Spock's primitive computer workshop is a small-scale version of this, including an entirely gratuitous Jacob's Ladder.
• Meaningful Name: In The Star Trek Compendium, Allan Asherman suggests that the name "Keeler" is derived from the "keel" of a ship, the longitudinal element of a vessel that keeps it held together – much as Keeler herself keeps the time continuum from coming apart. It also could be interpreted as a hybrid of "killer" and "healer" – a reference to her dual role as the focal point of the time flow. However, in Ellison's first treatment for this episode, Edith's last name was Koestler.
• Missed Him by That Much: McCoy is led out of the soup kitchen just as Spock shows up to serve.
• Necessary Fail: Kirk prevents Edith Keeler from being hit by a car, only to discover that she would go on to campaign against the US getting involved in World War II, which would in turn give Nazi Germany the opportunity to develop the atomic bomb and use it to win the war. He then realizes that he has to let her die in order to prevent millions more deaths.
• The Needs of the Many: Kirk falls in love with Edith Keeler, partly because she has the same ideals upon which Starfleet would later be founded. But if he allows her to live, millions will die who did not die before, and Starfleet will never exist. He is forced to let her die to restore the timeline.
• Nice Job Breaking It, Hero:
  • This would have been how history remembered Edith Keeler, had she been allowed to live. She was a saintly Depression-era social worker who, had she not been killed in a hit-and-run, would go on to lead a nationwide peace movement that would keep the U.S. out of World War II for several years beyond that of the original timeline — allowing the Nazis more time to develop nuclear weapons, ultimately win the war, and presumably cause The End of the World as We Know It. Nice job — wait, never mind.
  • Related to the above, Bones saving Edith from from the car accident that would have killed her, which sets off the domino effect that undoes the very future Bones came from.
• No Historical Figures Were Harmed: Edith Keeler was loosely based on Aimee Semple McPherson, whose biography Ellison was reading at the time. During the interwar years, McPherson was a popular evangelist, known to her fans as "Sister Aimee." If you've ever wondered why the credits refer to Edith as "Sister Edith Keeler," this is why. While McPherson really was an idealistic pacifist who advocated on behalf of the poor, she was also a controversial public figure embroiled in various scandals. Edith also seems to lack McPherson's more religious views, such as her advocacy of faith healing, which she personally practiced on her followers. On the other hand, McPherson pragmatically abandoned pacifism when World War II came around, so at least she has that over her fictional counterpart.
• Not Even Bothering with the Accent: Joan Collins, who's British, didn't bother attempting an American accent while portraying Edith Keeler, though it's highly possible that she's an Englishwoman who immigrated to America at one point.
• Not That Kind of Doctor: Inverted when, upon hearing that Edith has a friend "who talks about Earth in the same way you do" and offers to introduce him, Bones tells her, "I'm a surgeon, not a psychiatrist."
• Oblivious Guilt Slinging: Downplayed; McCoy is knowingly calling Kirk out for stopping him from saving Edith, but he doesn't know why or how much it tortures Kirk to do it.
  McCoy: You deliberately stopped me. Do you know what you just did?
  Spock: He knows, Doctor. He knows.
• Obstructionist Pacifist: Edith Keeler is one of these. There's nothing wrong with her ideas of world peace and humanity working together to improve themselves (if anything, it draws Kirk's eye even more, coming from a time where views like hers have actually come true). But unfortunately for the rest of the universe, this is not a mentality that is good to have (or force the U.S. to have) in the middle of World War II.
• O.O.C. Is Serious Business: Scotty and Uhura are initially happy to see the guys return, but notice that Kirk is stuck in a Thousand-Yard Stare.
• Pop-Cultural Osmosis Failure: Bones is well-aware of what movies are, but hasn't a clue who Clark Gable is. Kirk is likewise unfamiliar with him, which makes Edith mention Bones and sets off the episode's conclusion.
• Portal to the Past: The Guardian of Forever can not only display recordings of events at any time in Earth's past, but also allow the interested explorer to travel back to those times.
• Precision F-Strike: "Let's get the hell out of here." The effect is obviously lost for modern viewers, but "hell" was pretty shocking for 1960s television (contrast Kirk's slightly awkward line "Go to the Devil" from Day of the Dove), and they had to fight the network watchdogs to keep it in.
• Psychic Powers: The Guardian of Forever remains cryptic about its true nature (to Kirk's question "Are you machine, or being?", the Guardian answers, "I am both - and neither."), but it can communicate with the Enterprise landing party despite having no mouth or ears.
• Reckless Gun Usage: After the still heavily-drugged Bones passes out in the middle of a paranoid rant to an alarmed bum, the bum searches his pockets for money or valuables and finds his phaser. He inspects the strange device and presses a button on it - while pointing it at himself. He is promptly disintegrated.
• Re-Release Soundtrack: "Goodnight, Sweetheart" was replaced with generic music for monetary reasons on the VHS and Laserdisc releases, but the track was accidentally included on the DVD release, so Paramount paid the money and all future releases kept the track.
• Reset Button:
  • According to the Guardian, if they were successful in restoring the timeline, "It will be as though none of you had gone". Then, this would indicate that the bum who accidentally killed himself with McCoy's phaser is restored to life. And from Edith's point of view, she never met any of them before she died. However, Kirk still remembers.
  • The Guardian's planet, or presence, acts as a temporal shield, as anyone on it during a Time Change is immune to the effects.
• Right Man in the Wrong Place: Spock agrees with Keeler's visionary beliefs that peace was the way, but he feels it was at the wrong time. He considers that her ideals may have had better appeal and success in a post-war era rather than in the middle of an era of depression and global conflict.
• Ruins for Ruins' Sake: The planet inhabited by the Guardian of Forever is covered by ruined stone columns. What they are doing there is never explained.^note  The use of this trope is accidental; the script describes the planet as being covered in ancient runes.
• Save This Person, Save the World: Inverted; in this case, it's Kill This Person, Save the World. If Edith Keeler lives, the Nazis win World War II, and Starfleet is never founded. If she dies, history continues as normal.
• Screw This, I'm Outta Here!: At the end of the episode, the Guardian of Forever offers to send the Enterprise crew on many journeys throughout history; the traumatised Kirk immediately opts to "get the hell out of here".
• Sequel Hook:
  • The Guardian's final words definitely give the impression that the writing staff intended to make further historical stories. Unfortunately, it never happened - at least in this series. It gets a proper sequel in the animated series episode "Yesteryear".
  • The Guardian — or its big brother — gets a further work-out in the fan-made episode "In Harm's Way".
  • The Guardian reappears in "Terra Firma" parts 1 & 2 of Star Trek: Discovery.
• Set Right What Once Went Wrong: Inverted when Bones goes back to the past and prevents Edith Keeler's death, thereby preventing Starfleet from ever existing, then played straight when Kirk and Spock have to follow him and prevent him from preventing Edith Keeler's death.
• Shoot the Shaggy Dog: Much of the episode focuses on Kirk's growing romantic attraction to Edith. But at the end of the episode, he is forced to set his feelings for her aside and let her be hit by a car and killed in order for the timeline to be restored.
• Slasher Smile: Bones briefly has one.
• Soapbox Sadie: Edith Keeler subjects the regulars at the 21st Street Mission to numerous speeches about the greatness of which humanity is capable, and the notion that one day, they will harness atomic energy, travel into space, cure disease, and feed the hungry. Most of the poor and hungry diners see her speeches as the price they have to pay for a hot meal at the mission. Kirk, however, is fascinated by Keeler's ideals, which are almost identical to those on which the Federation and Starfleet would be founded three centuries later. There's just the small problem of another couple of World Wars in between...
• Status Quo Is God: Neither Spock nor Kirk suggests the possibility of taking the forward thinking Edith Keeler back to the future with them instead of letting her get killed 'again'. Perhaps this is what the "Guardian" is guarding against. (There appears to be no clear means whereby this could have happened.)
• Stock Footage:
  • The Guardian of Forever shows history via stock footage from old films and newsreels. One shot of a sailing ship firing, taken from the 1945 Swashbuckler The Spanish Main, was later used twice on Star Trek: Enterprise, in the episodes "Storm Front" and "In a Mirror, Darkly." Footage of a Nazi rally is taken from the 1935 German propaganda film Triumph of the Will.
  • Stock footage is used for the establishing shots of 1930 New York City. A shot of the Brooklyn bridge is taken from the 1949 musical On the Town. Another shot features a fallout shelter sign, which is obviously anachronistic.
  • A reaction shot of Kirk is recycled from "Dagger of the Mind", and a reaction shot of Spock from "The Naked Time" when the crazed McCoy wreaks havoc on the bridge.
• The Story That Never Was: The episode ends with this trope implied. When McCoy gets drugged and falls through a mysterious portal through time, his changes to the timeline cause the Enterprise to disappear from the present. Kirk and Spock follow McCoy into the portal, arriving in New York City in The '30s. They meet the peace activist Edith Keeler, and soon realize that she's the change in the timeline. McCoy saved her (or will soon save her) from an early death, and her activism will delay the USA's entry into World War II just long enough to lead to a Bad Future where Nazi Germany wins. So Kirk prevents McCoy from rescuing Keeler—erasing McCoy's involvement in the past and restoring the timeline to normal.
• Sufficiently Advanced Alien: Either the Guardian itself, or whoever built it.
• Surprisingly Realistic Outcome:
  • Kirk and Spock travel back in time to Depression-era New York in order to avert a disastrous event that changed history, but they don't know what it is. Fortunately for them, the information is stored on Spock's tricorder. The tricorder works, but the stored information must first be downloaded into the ship's computer, which doesn't exist yet. Spock tries to create a compatible computer, but he quickly finds it almost impossible. No matter how smart Spock is, the tricorder and Star Fleet's computers are centuries ahead of the most cutting-edge technology available at the time, and he's been trained to use highly advanced tools in a time where getting a pound of pure gold or platinum for your amateur electronics project is no simple matter - he compares it to working with "stone knives and bearskins". He's reduced to working with consumer-grade electrical goods such as lightbulbs and radio sets, and can only get a few seconds of functionality out of the homemade components after days of work. Also, in order to buy those materials he and Kirk need to work menial odd jobs and live in a homeless shelter, and in order to fit in they steal clothes off a clothesline... where they are promptly confronted by a police officer.
  • McCoy changes the past such that Germany won WWII, drastically altering Earth's history such that there's no Federation or Starfleet and thus no Enterprise waiting in orbit. This is far more plausible than the way these stories are widely played, where a Nazi Federation arose and somehow there's still an Enterprise in orbit but it's run by Nazis.
• Time Travel Escape: This was clearly an Unbuilt Trope, as this is not even suggested as a way of preventing Edith Keeler's death. It is speculated that this may be what the "Guardian" is guarding against, as it clearly does not prevent altering history by other, more conventional means.
• Time-Travel Romance: Kirk and Spock travel back in time by several centuries, where Kirk falls in love with "temporal local" Edith Keeler.
• Time Travel Taboo: The Federation is nearly erased when Dr. McCoy unwittingly changes history by saving a 1930s peace activist who delays the USA's entry into World War II, enabling Nazi Germany to win the conflict and Take Over the World. After this situation is resolved, the planet is placed under strict quarantine.
• Too Good for This Sinful Earth: Edith Keeler is warm, compassionate, idealistic, has not an unkind thought for anyone and dreams of a better future — and she has to die in order to preserve the timeline. In fact, it was her goodness in a world that couldn't yet live up to her ideals that would have resulted in disaster, making her a more literal example than most.
• Tragic Time Traveler: Dr. McCoy accidentally travels back in time to The '30s and manages to make the entire Federation become Ret-Gone. Kirk follows him with a landing party and falls in love with a local woman named Edith Keeler, whose life Bones saved from a car accident. Unfortunately, it turns out this was a linchpin event that led to Nazi Germany winning World War II, because in the altered timeline she led a pacifist movement that kept the United States out of the war. The crew are forced to let her die to restore the timeline.
• Unwitting Instigator of Doom: Both McCoy and Edith Keeler; Bones saved an innocent woman from being hit by a car, and Edith, in the altered timeline, worked for the noble cause of peace, but in doing so, they inadvertently allowed Nazi Germany the time to develop the atomic bomb and win World War II.
• We Are Not Going Through That Again:
  Guardian of Forever: Time has resumed its shape. All is as it was before. Many such journeys are possible. Let me be your gateway.
  Uhura: Captain, the Enterprise is up there. They're asking if we want to beam up.
  Kirk: Let's get the hell out of here.
• Wham Line: "Edith Keeler must die."
• What the Hell, Hero?: Bones, after Kirk restrains him from saving Edith.
• Year Inside, Hour Outside: When Kirk and Spock re-emerge from the Guardian's portal after having spent a decent chunk of time in 1930, Scotty asks, "What happened, sir? You only left a moment ago."
• You Can't Fight Fate: Edith Keeler must die so that Germany doesn't win World War II and wipe the Federation from existence. (Had she lived, she would have founded a peace movement that would have delayed the United States' entry into the European front of WWII, allowing Nazi Germany sufficient time to develop the atomic bomb and thus win the war.)
• You Get What You Pay For: One of the regulars at the soup kitchen claims having to listen to Edith's soapbox is the price for a free meal.
• You Have GOT to Be Kidding Me!: Spock's reaction clearly reads this when Kirk tries to pass off Spock's ears as being caused by a mechanical rice picker accident.