Worf returns home from a Klingon bat'leth tournament that he won to a surprise birthday party thrown by Riker. As he celebrates being "old enough", he notices several subtle changes in his own reality: The flavor of the cake changes, Captain Picard is at the party when he said he wouldn't be available. As the Enterprise works on the Argus Array, a space telescope that is having problems, Worf has more dizzy spells, with more slight perception problems: officers switching positions, appearing and disappearing. More troubling, Doctor Crusher remembers him complaining about a concussion he suffered when he lost the bat'leth tournament. He returns to his room to find his champion's trophy replaced with one for ninth place.
More and more things Worf has been party to are being forgotten by his crewmates, while things he doesn't remember are being claimed by them. He claims the Array was being tampered with by the Cardassians, but no one believes him. Angry, he takes himself off duty, and Troi tries to comfort him. As La Forge enters his quarters to tell him what they found with the array, he blacks out twice. The second time he suddenly finds himself in the middle of a battle situation with Cardassians and he tries to shake off the effects and figure out how to operate the new controls. The disorientation proves costly, as the attack badly injures La Forge and it seems likely that his delay was the reason for this. Worse, when he's relieved of duty for his "memory loss", he now doesn't have any bat'leth tournament trophy, as he apparently skipped it for repairs to the ship. Another shock comes in the form of Troi, who announces she is his wife. Worf has Data try to find any temporal anomalies that might explain the changes, but the now blue-eyed android can find none, so they search for commonalities in the events. Worf figures out that La Forge was present in all of them. As they head down to sickbay, the doctor (Ogawa instead of Crusher) informs them that Geordi is dead.
Geordi's body gives no indication, so they check his visor...and Worf gets dizzy again. When he comes to, he's in a red command uniform and is informed that he's the first officer of the Enterprise under Captain William Riker, ever since Jean-Luc Picard was killed in the Borg incident (a deviation from what happens in "The Best of Both Worlds"). Data scans Worf and finds what Worf is saying himself: He doesn't belong in this reality, where the tactical officer is Lieutenant Wesley Crusher and the conn officer is a Cardassian. A search for anomalies finds a quantum fissure in the space-time continuum that was crossed by Worf's shuttle in his own timeline. As the Enterprise scans the fissure, Worf returns to his quarters with Troi, who again is his wife, and they have two children in this timeline, but Alexander doesn't exist. The scan is interrupted by a Bajoran warship (as the Bajorans turned warlike after defeating the Cardassians, who joined the Federation), and the battle disrupts the fissure; hundreds of thousands of Enterprises appear out of nowhere.
Riker sends out Worf's genetic info to the Enterprises, as it has the signature of his own reality encoded in it. They find the Enterprise that matches his information, and sends a shuttlecraft to get him. However, before he can get to the fissure and seal it, another Enterprise starts to fire on it. This Enterprise is from a world where the Borg are everywhere, and a disheveled and panicking Captain Riker proclaims they won't go back. The "current" Enterprise tries to disable it with a photon torpedo, but it is so damaged the shot destroys it. Worf enters the rift, fixing the anomaly. He finds himself in his own uniform, with a Bat'leth tournament Champion's trophy. This time when he returns to his room, only one person is waiting for him: Deanna Troi. Thinking about the Trois of the other universes, he decides to invite her to a birthday dinner for two.
- Adorkable: A rare moment for Worf when Deanna makes herself at home in his quarters (with him not knowing that they're married). The icing on the cake is a visibly confused (and nervous) Worf peering into his bedroom while Deanna is on his bed.
- Ageless Birthday Episode: When asked how old he is, Worf simply says "Old enough."
- Alternate Reality Episode: A whole bunch of them.
- Beard of Sorrow: Captain Riker from the Borg-controlled reality has a massive unkempt beard and looks noticeably dishevelled from all the stress.
- Book-Ends: Worf returning to the Enterprise after a bat'leth tournament and going to his quarters, talking with Riker along the way, suspecting a surprise birthday party waiting for him. In the beginning part of the episode, the birthday party takes place, but in the ending, it doesn't, with just Deanna Troi taking care of things until Worf returned.
- In one universe, Data explains that Worf's marriage to Counselor Troi was rooted in her helping him recover after his spinal cord injury in season 5's "Ethics".
- The last universe is established as being how things would've been had they been unsuccessful in saving Picard from the Borg in "Best of Both Worlds". One of the many Enterprise copies seen at the climax is implied to be from a reality where they couldn't save Picard or stop the Borg assimilating Earth.
- Troi alternates between much of her wardrobe throughout the series as we shift from universe to universe.
- The Argus Array previously appeared in season 4's "The Nth Degree," and is revisited here presumably as an excuse to re-use the effects shots. While re-using effects shots like that isn't at all uncommon for Trek, it's much rarer for them to actually revisit a location, instead of just trying to pass it off as a different, but identical-looking place.
- Captain's Log: Worf records a log entry on his way back to the Enterprise, which becomes a plot point when it changes between quantum realities.
- Cassandra Truth: How Worf feels about nobody believing him when he points out that things aren't right. At least until Data finds the Technobabble that confirms his story.
- Chekhov's Gun: Geordi's VISOR triggers Worf's various shifts.
- Crapsack World: One Enterprise comes from a reality where the Borg successfully conquered the Federation in "The Best of Both Worlds". The Riker from that universe is so broken that he'd rather kill Worf than return to his universe. Of course, when the native timeline's Riker disables that Enterprise, it's already so damaged that it immediately explodes.
- Crapsaccharine World: The last reality Worf finds himself in. He's the first officer of the Enterprise, and married to Troi with two children. But Alexander isn't one of them. And Picard and Geordi are dead.
- Didn't Think This Through: When Worf asks Deanna to become Alexander's surrogate mother, legally available to care for Alexander should anything happen to Worf. Troi asks what that would make her to Worf, and he says the Klingon word for this relationship has no exact translation but the closest equivalent is that she would be his stepsister. Deanna's response? "That would make my mother your stepmother." The look on Worf's face...Worf: I had not considered that... (beat) It is a risk I am willing to take.
- Doppelgänger Gets Same Sentiment: The Primeverse Enterprise briefly receives a transmission from an alternate-universe version of Riker, now in command after Picard died during the events of "The Best of Both Worlds". He expresses his happiness to see Picard, saying that it's been a while.
- Expendable Alternate Universe: To such a degree that J. J. Abrams used this episode to explain how the new timeline from the 2009 reboot works with the Prime timeline.
- Foreshadowing: At the beginning, Riker is accompanying Worf to his quarters, and when Worf mentions it's his birthday and dreads that Riker or someone else planned a surprise party, Riker responds in disgust that he hates surprise birthday parties and would never do it to anyone else. Later when it turns out there is indeed a party Riker slaps a goofy party hat on Worf and says "I love surprise parties". In the offset it just seems like Riker was messing with him, not the result of a shifted reality.
- Funny Background Event: When Captain Picard starts talking with Captain Riker, Commander Riker shares a surprised look with Lieutenant Worf over seeing Captain Riker and Commander Worf.
- A Glitch in the Matrix: Worf begins to notice that small details are off from how he remembered them — the placement of decorations, the positions where people are standing, Starfleet uniforms and insignia, and even Data's eyes changing colour. As the episode progresses, the differences are magnified to the extent that Picard was killed at Wolf 359, Riker is captain of the Enterprise, and Counselor Troi is Worf's wife. Rather than being trapped in a Lotus-Eater Machine, it turns out that Worf is shifting through increasingly divergent parallel universes.
- Guess Who I'm Marrying?: A heroic variant when Worf finds out he's married to Troi in one universe.
- Happy Birthday to You!: The crew sings "For He's A Jolly Good Fellow" in Klingon. Worf is not amused.Worf: That is not a Klingon song.
Troi: It wasn't easy to translate. There doesn't seem to be a Klingon word for "jolly".
- The Hero's Birthday: Worf's birthday.
- Hero of Another Story: Captain Riker and the crew of the Enterprise in the last timeline that Worf visits. Their adventures after Wolf 359 could make for an interesting series.
- A collection of short stories examining "what if" scenarios implied that it didn't go that well and there may be some heavy darkness just outside of the veneer of normalcy we see... Riker, still somewhat reeling from the loss of Picard, was more willing to listen to the rogue captain in The Wounded and quite possibly reignited the Federation/Cardassian war.
- He Who Fights Monsters: The Bajorans, in one timeline.
- Human Mom, Non-Human Dad: Worf and Deanna's unseen children in the final alternate timeline. Their mother's half-human, half-Human Alien; their father's a full-on Rubber-Forehead Alien.
- In Spite of a Nail: All the universes Worf shifts through are at least broadly similar to the "real" one, in that he and most of the same group always end up on the Enterprise. (This is vaguely implied to be an actual in-universe phenomenon, as for the most part Worf's brief exposures are "nudging" him into timelines that are relatively close to his own, with only a few minor divergences in most cases.)
- It's a Long Story: Worf to Troi while confirming he's in the correct universe.
- Killing Your Alternate Self: Captain Riker gives the order to fire on the Crapsack World Enterprise to stop them from attacking Worf. He only intended to disable them, but that Enterprise is in such bad shape that one hit destroys them.
- Mythology Gag: The commbadges in the last reality visited by Worf are the same as the false future seen by Riker in "Future Imperfect".
- Other Me Annoys Me: Captain Riker is very disturbed at seeing a panicked, disheveled version of himself.
- Real Life Writes the Plot: It turns out that rights to the song "Happy Birthday", despite its common use, are privately held. To avoid any sort of legal issue (similar to why it took a long time before Moriarty returned to the series), the writers thought it safe to substitute "For He's a Jolly Good Fellow" instead, despite the rights owners not really seeing this as an issue.
- Revealing Continuity Lapse: Worf notices these increasingly as the episode progresses. Each time something changes, he's in a new quantum reality.
- Ripple Effect-Proof Memory: Worf apparently remembers everything that happened to him in the alternate quantum realities, though nobody else has any idea that anything happened until he says so.
- Screw This, I'm Outta Here!: The Bajoran ships attacking an alternate universe Enterprise tuck their tails and run when they see thousands of Federation ships descending on them.
- Ship Tease: The seeds for this season's Worf/Troi tease really begin here. According to Brannon Braga, the writers had actually been thinking about this angle since Season 5.
- Spoiler Title: If you're familiar with the concept of parallel universes, the title of the episode makes it easy to figure out what's going on long before the characters do.
- Spotting the Thread: "The cake was chocolate, but now it's yellow..."
- Stock Footage: The only new visual effects shots for this episode are the shots of the fissure, and the shots involving thousands of Enterprise clones showing up. Probably because of the time and expense involved in creating the latter shot, every other effects shot is taken from previous episodes.
- Surprise Party: For Worf, but only in certain of the timelines. At the end of the episode, it turns out that in the prime reality Riker wanted to throw the surprise party, but Troi talked him out of it. So Worf and Troi share champagne instead.
- Wham Line: After Worf is stunned by Troi kissing him.Worf: I do not believe this is appropriate behavior!Troi: Not even for your wife?