Two agents from Temporal Investigations, Luscly and Dulmer, arrive on the station to interrogate Sisko on his recent excursion in time. Sisko explains that it all began two weeks ago, when he traveled on the Defiant to Cardassia to fetch the Orb of Time, which the Cardassians are returning to Bajor. We flash back to the ship taking on the orb as well as a human passenger, Barry Waddle, who was stranded on the planet during the Klingon attack. Everything is going fine until a sudden surge in chroniton radiation envelopes the ship. It drops out of warp, decloaks, and transports someone away. When the viewscreen comes on, they recognize the original Constitution-class Enterprise orbiting a space station.

They are 105 years in the past, and they've identified that their passenger, Waddle, is actually a Klingon spy named Arne Darvin whose life went south after he failed a mission during the events of "The Trouble with Tribbles". Darvin has used the Orb of Time to get a second chance at his mission by assassinating James T. Kirk before he can expose Darvin's younger self. The crew have to stop him, so they get dressed in the suspiciously 1960s fashion of the 23rd Century and covertly beam over to find Darvin.

O'Brien and Bashir bumble their way through the old-fashioned technology of the original Enterprise while Odo lounges in the bar, where he discovers that Darvin has been through recently and tried to order a raktajino. While awaiting Darvin's return, Odo watches Uhura receive a tribble from Cyrano Jones. Worf arrives and discovers that Odo has taken a tribble as a pet. Worf bristles and describes the thing as an ecological menace that the Klingons ended by exterminating them long ago. Meanwhile, a Klingon D7 battle cruiser drops out of warp, and Dax identifies it as her old friend Koloth's ship. Dax recalls that Koloth once traded insults with Kirk but never faced him in battle, so he's just here for shore leave.

Bashir and O'Brien bump into the attractive Lt. Watley for a second time, and she propositions Bashir. The doctor realizes that she might be his great-grandmother, and he wonders whether he has to become his own great-grandfather or cease to exist, but he lets it pass. While Dax and Sisko gawk at Kirk and Spock, Bashir and O'Brien join Odo and Worf in the bar to spy on Scotty, Chekov, and Freeman. They also notice a group of Klingons who look nothing like 24th Century Klingons, and Worf doesn't want to discuss it. The Klingons pick a fight with the famous Enterprise crew, and our own characters get wrapped up in the fray. Afterwards, Kirk chews everyone out, and O'Brien is one of those singled out.

During the fracas, Worf has managed to capture the older Darvin, who admits that the method of Kirk's impending assassination will have a certain poetic justice to it. Indeed, with Odo's help, they figure out that Darvin has put a bomb in a tribble as revenge for Kirk using a tribble to expose him. The bomb will blow in an hour, so they'd better start scanning tribbles, whose population has of course exploded by this point. On the bridge, Dax recognizes Dr. McCoy and implies that a previous host once had a dalliance with him.

On the station, the team is frantically scanning tribbles in the grain storage area, where Dax has detected the bomb-laden tribble is located. All these tribbles have died due to Darvin's poisoned grain. Kirk and Spock open the compartment and are in the middle of uncovering Darvin's plot when the bombed tribble is located. Our heroes beam it outside, where it explodes harmlessly, saving the day.

Sisko ends his story by stating that history progressed just as it was supposed to, with Kirk using a tribble to expose the younger Darvin as a Klingon spy. However, Sisko does admit that he fulfilled his fantasy of telling Kirk that it was an honor to serve with him. Kirk asked his name, smiled, and told him to carry on. Sisko states that he'll accept a reprimand for his actions but doesn't regret them. Surprisingly, Dulmer brushes it aside and assures Sisko that his report will be fine. However, the investigators never asked about the tribbles. It turns out that Odo brought his tribble back to the present, and now the station is infested with tribbles.

Tropes:

• Ascended Fanboy
  • Dax may be the most outwardly enthusiastic, but don't let that fool you. The other Starfleet officers are just as thrilled to be there and to see Kirk in action.
    Sisko: Excuse me, Captain. Here's tomorrow's duty roster for your approval.
    Kirk: Lieutenant... uh, Lieutenant?
    Sisko: Benjamin Sisko, sir. I've been on temporary assignment here. Before I leave, I just want to say... it's been an honor serving with you, sir.
    Kirk: All right, Lieutenant. Carry on.
    Sisko: Thank you, sir.
  • O'Brien stands in line as Kirk dresses down the brawlers.
    O'Brien: I lied to Captain Kirk! I wish Keiko could have been there to see it.
• Aside Glance: Sisko gives a priceless one when Jadzia mentions that Bones McCoy has "the hands of a surgeon".
• Author Avatar: Dax's enthusiasm for the time period of the original series was a direct reflection of the writers' love for the old show. It helps that Dax's actress, Terry Farrell, was a Promoted Fangirl of TOS.
• Back to the Early Installment: The plot sees the crew visit the original Trek episode "The Trouble with Tribbles".
• Bait-and-Switch: An absolutely delighted Jadzia, watching the bridge crew, comments to Sisko "He's so much more handsome in person." When Sisko responds that Kirk always had a reputation as a ladies' man, Jadzia responds that she's actually talking about Spock.
• Bar Brawl: One of the most famous scenes in both tribble episodes after a Klingon hits a certain Scotsman's Berserk Button by insulting the USS Enterprise. This time, O'Brien and Bashir get mixed up in it.
• Been There, Shaped History: Dax met McCoy sometime in the past when she was still bound to Emony. (Based on her comment about him having "the hands of a surgeon", they may have shaped more than history.)
• Blatant Lies: The post-fight "I don't know, sir" bit is preserved, but with O'Brien replacing Freeman.
  Bashir: That was close.
  O'Brien: Me. Of all the people in the lineup, he asks me who threw the first punch.
  Bashir: And you lied to him.
  O'Brien: I lied to Captain Kirk! I wish Keiko could have been here to see it.
• Brick Joke:
  • Odo buys a tribble from Cyrano Jones, clearly not knowing anything about it. At the end of the episode, the Promenade is swarming with tribbles.
  • We find out why Kirk was getting pelted with individual tribbles during the original episode: Dax and Sisko were tossing tribbles out of the way to scan for the one with the hidden bomb.
• Call-Back:
  • The episode makes a number of references to old TOS episodes.
  • Two to the earlier Deep Space Nine time travel episode "Past Tense" — Sisko knows his history quite well, but Bashir doesn't.
  • Mention is made of the "duotronic" sensor technology of the era, which is really a call-back to the TOS episode "The Ultimate Computer" in which the Enterprise served as a test-bed for brand-new multitronic computer tech.
• The Cameo: David Gerrold, the writer of "The Trouble With Tribbles", requested a cameo as a condition of his endorsement for the episode. He makes two brief appearances as an ''Enterprise'' security guard; in his second appearance, he's shown petting one of the tribbles manufactured for the original episode.
• Canon Marches On: When Dulmer and Lucsly ask Sisko which Enterprise the crew came across when they travelled back in time, Sisko says "the first", and it's implied all three agree he's talking about the NCC-1701, i.e. Kirk's Enterprise. However, a few years after this episode Star Trek: Enterprise would retcon Star Trek canon so that the first Enterprise (the NX-01) actually existed a 100 years before the NCC-1701, and later Star Trek shows have made it clear that the NX-01 is still very well known during the era Deep Space Nine is set in. So, in retrospect, it doesn't make much sense that three Starfleet officers well versed in history would think the NCC-1701 was the first Enterprise.
  • The best explanation is that they are referring to the NCC-1701 as the "first" Enterprise in context of being the first Federation ship to bear the name, while the NX-01 was a United Earth ship.
• Cerebus Retcon: Remember the "Everybody Laughs" Ending of the original episode when Scotty revealed that he beamed the Enterprise's tribbles to the Klingon ship? That resulted in an ecological disaster for the Klingon Empire and the Klingons hunting tribbles to extinction. Oops.
• Character Shilling: The DS9 cast spends quite a lot of time talking about how amazing the TOS characters are.
• The Comically Serious: Dulmer and Lucsly.
• Continuity Drift:
  • Dax recognizes Bones McCoy because she had met him as Emony even before he became a doctor, and also sees Spock for the first time. Later in the series, it's implied that Curzon was present at the Khitomer Peace Conference (though it's possible that they never actually saw each other at Khitomer).
  • The Temporal Investigations agents have a brief aside about the number of Starships Enterprise, where Lucsly firmly states that there have been six. This was meant to be a reference to the newly-constructed U.S.S. Enterprise-E, which was due to make its first appearance in Star Trek: First Contact. The eventual creation of the Enterprise NX-01 would make it rather unclear which Enterprise Lucsly had been referring to.
• Continuity Nod: Dax really wants to see Koloth back when he was in his prime, and she's most disappointed by Sisko reining her in. Back in Season 2, it was established that Koloth was a close friend of Curzon's.
• Cuteness Proximity: To the tribbles, naturally. Even Odo is not immune to being infected with playful joviality by their presence.
  Odo: It's been my observation that most humanoids love soft, furry animals, [voice softens] especially if they make pleasing sounds.
• Damn You, Muscle Memory!: Played for Laughs. Sisko tries to contact the Defiant by tapping his Starfleet badge, before remembering that in this era they were just a cloth patch, and he has to use a handheld communicator. Given Dax's smirk behind him, she did remember.
• Deadpan Snarker: Odo, when questioning Worf about the Klingons' history with the tribbles, is particularly merciless with the sarcasm.
  Odo: Oh, another glorious chapter in Klingon history. Tell me, do they still sing songs of "The Great Tribble Hunt"?
• Death by Irony: Kirk used a tribble to expose Darvin as a Klingon, while the others had accidentally wrecked his plan to poison the quadrotriticale. Darvin's plan to kill Kirk with a bomb placed inside a tribble, saying it will be a most poetic end.
• Death Glare: As they leave the station, Lucsly can be seen scowling at Dulmer for approving of Sisko meeting with Kirk.
• The Dreaded:
  • As far as Dulmer and Lucsly are concerned, the otherwise heroic James T. Kirk is this, courtesy of his record of temporal violations.
    Dulmer: His ship.
    Lucsly: James T. Kirk.
    Sisko: The one and only.
    Lucsly: Seventeen separate temporal violations. The biggest file on record.
    Dulmer: The man was a menace.
  • And Dulmer and Lucsly, or more specifically Temporal Investigations in general, are this themselves.
    Sisko: We'll have to search both without arousing suspicion or altering the timeline ourselves. The last thing I want is a visit from Temporal Investigations when we get home.
  • The tribbles are considered an ecological disaster by the Klingon empire after the events of "The Trouble with Tribbles". Worf is put on edge by the sight of one.
• Eating the Eye Candy
  • Bashir, Sisko, and O'Brien gaping at Dax's miniskirt.
  • Bashir and O'Brien are checking out Lt. Watley as she walks away from the turbolift.
  • Dax sighs over Spock's beautiful eyes.
  • It's easy to read this into Uhura as she gazes at "Lieutenant" Benjamin Sisko while he's introducing himself to Kirk at the end. (In the original Stock Footage, she was marveling at the arrival of Prime-Universe Marlena Moreau).
• Establishing Character Moment: A retroactive example with Dulmer, which actually comes at the very end of the episode when he admits to Sisko that he probably would have said hello to Captain Kirk himself if he had been in his shoes. His comment, along with Lucsly's reproachful reaction, sets them up for a Red Oni, Blue Oni dynamic when they go off to star in their own novels. (Lucsly, by contrast, is every bit as stern as he appears during his introduction.)
• Fanservice with a Smile:
  • The waitresses at the bar, in keeping with the original show.
  • Dax gets into the act while showing off the 23rd century miniskirt.
    Bashir: I think I'm going to like history.
• Funny Background Event: During the final pull-out shot of Quark surrounded by tribbles, Morn can be seen in the background holding one and talking to it.
• Fun with Acronyms: One that's been lying in wait for 30 years. When O'Brien and Bashir are poking around the wall access panel, a pipe in the bottom is clearly labeled "GNDN435". This duplicates rarely-seen-on-camera labels created by TOS designer Matt Jefferies. "GNDN" stands for "Goes Nowhere, Does Nothing".
• Future Imperfect: A mild internal example, with some of the DS9 crew not knowing things about this era, such as the Starfleet division colours being different, and what Captain Kirk looks like, and even that you have to grab the hand-holds before the turbolift will respond to voice command.
• The Gadfly: After Darvin says that Klingons are smelly, O'Brien and Bashir says that they quite like Worf's smell, which is peaty with a touch of lilac. O'Brien later tries to get Dax to say that she smells lilac around Worf, but she says that she has her own ways of tormenting him.
• Gilligan Cut: From Sisko's testimony to Quark's Bar, revealing that a tribble or fifty had hitched a ride back to the present day.
• Hand Wave: Upon seeing 23rd century Klingons, the crew asks Worf about the ridges (or lack thereof).
  Worf: "We do not discuss it with outsiders."
• Head-Turning Beauty: Dax elicits this response from Bashir and O'Brien when she reveals herself in the 23rd Century miniskirt.
  Sisko: In the old days, operations officers wore red, command officers wore gold...
  Dax: And women wore less.
  [Dax enters and everyone gawks at her as she performs a Girly Skirt Twirl]
  Bashir: I think I'm going to like history.
• Humiliation Conga: When his plan was foiled and his true identity exposed, Darvin's life and career were ruined. Cast out by his own kind, he spent the following years as a mere merchant and had to pretend to be human. By the time of the episode proper, the Klingon invasion of Cardassia left him trapped on the wrong side of the border.
• Hypocrite: Sisko vetoes Dax's wish to meet the past incarnation of a Klingon friend of Curzon's... only to make sure to take the time later on to meet James Kirk himself.
• Hypocritical Humor: Invoked by Darvin, who makes a Fantastic Racism comment about Klingons. It's played for an I'm Standing Right Here gag (Worf is present), but it turns out that Darvin is a Klingon himself.
• I'm a Doctor, Not a Placeholder: Bashir is a Doctor, not an historian.
• Interrogation Flashback: This episode uses an interrogation by Temporal Investigations as a framing device, where Sisko explains how they got thrown back in time to the Star Trek: The Original Series episode "The Trouble with Tribbles".
• It's a Long Story: When Worf's crewmates see the QuchHa' crew of the IKS Gr'oth and can't believe that they're actually Klingons.
  Odo: Mr. Worf?
  Worf: [sighs] They are Klingons, and it is a long story.
  O'Brien: What happened? Some sort of genetic experiment?
  Bashir: A viral mutation?
  Worf: We do not discuss it with outsiders.
• The Knights Who Say "Squee!": The episode is MADE of this.
• Let Us Never Speak of This Again: In a 23rd century turbolift, O'Brien tries a voice command, but nothing happens. He and Bashir are completely baffled by this, until seeing Lt. Watley grabbing the hand-hold that makes the turbolift work. Bashir suggests he and O'Brien not mention this awkwardness to anyone ever.
• Meet Your Early-Installment Weirdness: "Those are Klingons?"
• Ludicrous Precision: When Sisko tells them that he time-traveled to when Captain Kirk was on K-7, the temporal agents instantly know how long ago that was, and even provide the day of the week.
• Make Wrong What Once Went Right: Darvin's days in Klingon intelligence ended once Kirk exposed him and his plan, leaving him as a mere merchant and a footnote in history. Upon learning of the Orb of Time, he attempts to change history by killing Kirk before that fateful moment. Sisko and company spend the episode trying to find Darvin, figure out his exact plan, and foil it.
• Mandatory Line: Quark is only seen at the very end of the episode looking annoyed at the infestation of tribbles in his bar.
• My Own Grampa: Lampshaded when Bashir encounters an attractive woman who has the same name as his great-grandmother. He starts panicking over the possibility that he is his own great-grandfather and that he could cease to exist if he doesn't get into her pants, but O'Brien tells him that he's being ridiculous.
  Bashir: I can't wait to get back to Deep Space Nine and see your face when you find out that I never existed!
• Mythology Gag:
  • At the K-7 bar, O'Brien mistakes a Starfleet officer for Kirk. The actor who plays the officer was William Shatner's stunt double on TOS.
  • The crew is surprised when they see TOS-era Klingons, since they lacked their trademark head ridges. When asked, Worf deflects the question, stating that they do not discuss it with outsiders.
  • When O'Brien looks into the panel in Engineering, he says that he's afraid to touch anything because it was all cross-wired and such (the work of Chief Engineer Scott). It was a legend of Star Trek that the only thing that was the same between the original technical manuals and Scotty's rewrites is that the manufacturer's warranty would be void if evidence of tampering was present.
  • Dax finds Spock hotter than Kirk. This may reference the fact that Spock was (and is) very popular with female fans, to the point Nimoy got more fan-mail than Shatner. ^note Shatner was a bit jealous, until Roddenberry talked him down.
• Needle in a Stack of Needles: Where did Darvin hide the bomb? Why, inside of a tribble...
• Nerds Are Sexy: Dax finds Spock very attractive.
• Never Heard That One Before: The Temporal Investigations guys to the time-related joke that Dax cracks.
  Dax: I guess you boys from Temporal Investigations are... always on time.
  [Dulmer and Lucsly exchange annoyed looks].
• No Sense of Humor: The Temporal Investigations agents have heard every single time-travel joke Eleventy Zillion times already, thank you very much. Their annoyed reactions to everyone else's repeated joking makes them Comically Serious.
• Not So Above It All: Not shown, but stated: when Sisko admits to speaking with Kirk before leaving the 23rd century, Dulmer mentions that he probably would have done the same thing. Lucsly clearly doesn't agree.
• Not So Extinct: By the time of DS9, the tribble species is extinct, having been wiped out by the Klingons. However, by the end of the episode, that's no longer true...
• Novelization: It was novelized by Diane Carey.
• Obstructive Bureaucrats: Played with. Temporal Investigations is apparently seen by most Starfleet personnel as people who show up just to make life hell for anyone who time travels, but the two that we meet are pretty reasonable guys. They do chide Sisko for some of his crew's temporal violations, but also assure him that there's little chance of any disciplinary action. The Strange New Worlds story "Gods, Fate, and Fractals" by William Leisner runs with this, with Dulmer and Lucsly regarding themselves as the Only Sane Men in a Kirk-worshipping Starfleet that keeps casually messing with the timeline to disastrous consequence.
• Oh, Crap!: Worf's reaction when he sees Odo's pet tribble. Odo, of course, takes the opportunity to needle him about it.
  Worf: I have completed my search of the primary habitat level— [hears purring and tenses up] What is that sound?
  Odo: Soothing, isn't it? [reveals the tribble] The bartender called it—
  [the tribble shrieks loudly]
  Worf: A tribble. [gets a "This Is Gonna Suck" face]
• Poetic Justice: Darvin claims that killing Kirk with a tribble-bomb is this.
• Production Foreshadowing: Dulmer says to Sisko that there have been five vessels named Enterprise, prompting Lucsly to correct him by saying "Six", referring to the Enterprise-E, which was about to debut in Star Trek: First Contact.
• Real Life Writes the Plot: Ira Steven Behr has said that the episode's premise was chosen because actor Charlie Brill, who played Arne Darvin, happened to come into the restaurant where the DS9 production crew was discussing different TOS homage possibilities. Behr has since joked this remarkable coincidence "proves" that God was a Star Trek fan (while Brill similarly joked he was glad he didn't go out for Chinese like he'd originally been planning).
• Reasonable Authority Figure: After listening to Sisko's account of the events, Dulmer and Lucsly conclude that no harm has been done, and indicate that their report will be a favorable one. (Though things might have gone differently if they'd known that the Defiant accidentally brought a formerly extinct species into the 24th century.)
• Recap by Audit: The episode is told via flashback as Sisko explains what happened to the Department of Temporal Investigations.
• Red Shirt: One of the few times where a TOS Red Shirt in mortal peril survives.
• Retraux: For the scenes in the past, the production crew intentionally used primitive equipment and lighting techniques to make the footage "mesh" with the original episode's. David Gerrold noted that the one part of the episode's TOS sets that was out of sync with the original sets was some orange mesh. The company that made the original mesh had gone out of business some twenty years prior, and it was Unobtainium by the time the episode was made.
• Scene of Wonder: Sisko and his officers are awestruck when they come aboard the original USS Enterprise.
• Significant Anagram: The Temporal Investigation agents are named Dulmer and Lucsly, anagrams of Mulder and Scully.
• Skin-Tone Disguise: Dax paints over her Trill markings so she can pass as a human.
• Stable Time Loop: Dulmer and Lucsly ask Sisko if he thought that their trip to the past was one of these (even calling it by its proper name, a predestination paradox), but Sisko says no.
  Dulmer: Good.
  Lucsly: We hate those.
• The Stakeout: When Odo learns from the waitress at the bar aboard K-7 that Darvin was there earlier, he decides to stick around in case Darvin returns.
• Starring Special Effects: A subtler example than most, with the "special effects" in question being limited to the seamless insertion of new actors into old footage à la Forrest Gump. When trying to get the episode approved, special effects director Gary Hutzel showed what appeared to be a clip from the original episode to Ron Moore and Ira Behr, then pointed out that he had digitally edited in an extra actor. Having no idea that the footage had been edited in any way, the two were stunned and greenlit the episode on the spot.
• Stock Footage: Most of this is taken from "The Trouble with Tribbles", of course, but there's also a little bit from "Mirror, Mirror" used for the scene in which Sisko meets Kirk.
• Tempting Fate: Played for Laughs. The episode opens with Sisko receiving a visit from Temporal Investigations, and in the flashback, Sisko says that the last thing he wants is a visit from them.
• This Is Gonna Suck: The reaction of both Temporal Investigations agents upon finding out that Sisko's time-travel hi-jinks also involve a certain Constitution-class vessel and her infamous captain.
• Time-Travel Tense Trouble: Odo while describing what's going on aboard Station K-7.
• Too Much Information: Sisko's reaction (as an Aside Glance) to Dax fondly remembering that McCoy "had the hands of a surgeon."
• Unobtainium: The above-mentioned orange mesh, the only part of the original sets that couldn't be acquired or fabricated to make the retro sets.
• Villain Ball: Darvin's plan to kill Kirk probably would have worked if he hadn't bragged about it to the Defiant crew while there was still enough time for them to stop it.
• Whole Episode Flashback: The framing story is Sisko explaining the episode's events to the Temporal Investigations agents.
• Worth It: Before returning to his own time, Sisko can't resist meeting Kirk face-to-face (although still undercover). Back on DS9, he tells the Temporal Investigations agents, "If you want to put a letter of reprimand in my file for that, go right ahead." Dulmer even admits that he probably would've taken the opportunity himself, much to Lucsly's annoyance.
• You Have Failed Me: Worf and Odo explain that after Darvin was exposed as a Klingon and the plan to poison the quadrotriticale was foiled, Darvin was expelled from the Klingon Empire (no doubt via discommendation) and forced to eke out an independent existence as The Stateless.