It began when he returned from the Parada system. In preparation for a peace conference about DS9, he underwent basic training for the security measures aboard the station. When he wakes up in his quarters, things are off. Molly refuses to touch him and Keiko seems nervous. It's also 5:30 AM. With the excuse of several essays to read, Keiko hurries off to daycare with Molly.
Shrugging off the oddness, he heads in for duty. Only to find an Ensign named DeCurtis repairing a system O'Brien was waiting to fix until Odo returned to the station. DeCurtis informs him that he was given direct orders from Sisko to do this. Fairly annoyed at being overstepped like this, he heads off to speak with him. Only to see Sisko and Keiko having a suspicious conversation outside the school.
Back in the present, O'Brien is 53 minutes from the Parada system when sensors detect the Mekong in pursuit. They can't catch him at warp, so he continues his log on the events of the past few days.
Arriving at Ops, the crew is acting suspicious as well. Sisko and Bashir are oddly forceful about him reporting in for a standard physical. In Sisko's ready room, he quizzes O'Brien on any information he has about the Paradas. All O'Brien can come up with is that they emit a foul smell when they're upset.
Assigning him to fix three pylons that have oddly broken down, O'Brien asks what Sisko was talking to Keiko about outside the school. He assures him that he was simply discussing Jake's slipping grades. Reporting for his psyhical, O'Brien is rather peeved that it's taking an exceedingly long time. Assuming everyone is acting weird and the overly long physical is because he's dying, Bashir quickly gives him a clean bill of health and lets him leave.
Heading back to work, he runs into Jake. Who inquires if O'Brien can help him with an upcoming science project. This little interaction turns odd when O'Brien mentions Jake's slipping grades, only for Jake to say his grades are just fine. Which means Sisko lied.
It just keeps getting weirder for him. Ensign DeCurtis opening a door he insists only Kira had access codes to. Jake suddenly falling ill and not being able to come over for help with his project. Keiko cooking his favorite meal and yet acting oddly distant.
Late that night, O'Brien pours through every conceivable scan he think of. From unknown chemical agents to telepathic activity. Coming to a dead end there, he goes through the station logs. Only to run across a suddenly restricted file that requires level 1 clearance. Which he has. And yet his security code is rejected.
After fiddling with the system to get into the logs, he's shocked to find his personal logs have been broken into. Odo returns shortly after this, and O'Brien reports this weird activity to him. Odo is on his side, advising him to not act suspicious while he investigates things.
Visiting Odo in his office, even he's acting weird now. And to compound things, Sisko and Kira advance on him with weapons drawn. Drawing a hidden flashbang from his sleeve, O'Brien scurries through the vents and transports himself off the station.
Getting in contact with Admiral Rollman, she orders him to give up and return to DS9. With even the highest ranks of Starfleet apparently corrupted, O'Brien can do nothing but seeks answers on Parada II.
Beaming down to the planet, O'Brien finds Sisko and Kira already waiting for him with two Paradas. Insisting he drop his weapon, he claims a nearby door holds all the answers to this weirdness. As he prepares to open it, O'Brien looks prepared to fire on him. Forcing the other Paradan to fatally shoot him.
Behind the door is Bashir...and O'Brien. The real O'Brien. The protagonist of the episode was a replicant designed to assassinate someone at the peace conference. But he was designed too well. The copy believed he was the real O'Brien. Dying on the cavern floor, he asks one thing.
- Fake!O'Brien: Keiko...tell her I love...
- Bluff the Imposter: Bashir asks the replicant O'Brien several questions under the guise of a routine physical, including one about O'Brien's parents. The replicant, however, snaps that Bashir knows full well that O'Brien's mother passed away two years ago.
- Butt-Monkey: Even replicants of O'Brien are destined to suffer.
- Call-Back: When visiting Quarks, he recalls the cancelled racquetball match from "Rivals".
- Die Hard on DS9: For the replicant O'Brien.
- Downer Ending: Why yes, you can have your mind blown and become utterly depressed at the same time.
- Gone Horribly Right: The O'Brien replicant was designed a tad too perfect.
- Possibly some Fridge Brilliance to this. Why did the clone behave just like O'Brien? Because O'Brien isn't a soldier, operative or diplomat who might have to make moral decisions and pick sides - he's just a maintenance tech with a wife and child. He just wants to get through the day and in the end go home to his family. It is when his family behaves oddly toward him that he suspects something - because he is a maintenance man who fixes broken things.
- How We Got Here: The episode opens with the replicant O'Brien fleeing the station, convinced that something's wrong with the rest of the station crew. Everything leading up to that point is told by flashback as the replicant records a personal log.
- Manchurian Agent: The replicant's role was to assassinate someone at the Paradan peace conference, though we are never told who.
- Must Have Caffeine: Replicant O'Brien orders "coffee, Jamaican blend, double-strong, double sweet" several times.
- O.O.C. Is Serious Business: In perhaps one of the most subtly depressing uses of this trope ever, the already suspicious O'Brien replicant seems to really realize something is truly wrong when Keiko is kind and considerate to him after a long off-station assignment.
- Plummet Perspective: When the replicant O'Brien drops his phaser down one of the Jeffries tubes.
- Red Herring: Quark quotes one of the Rules of Acquisition, but doesn't quite remember which one it is (he thinks it's "one of the high numbers"), even on though in earlier episodes he's always remembered the exact number. This seems to support replicant O'Brien's suspicion that the people aboard Deep Space 9 have somehow been changed or replaced, but in the end it turns out to be a complete coincidence.
- The shift in Odo's behavior (from supporting the replicant O'Brien to falling in with the rest of the senior staff) also seems to support something being wrong. But Odo had been off-station when Sisko learned O'Brien had been replaced, and once he came back Sisko likely explained what was really going on.
- Silent Running Mode: The replicant O'Brien does this with his runabout in the Parada system.
- Surprise Party: Replicant O'Brien briefly wonders if everybody's odd behavior is just a prelude to one of these, which wouldn't make sense as his birthday's not for another few months.
- Tomato in the Mirror: The O'Brien replicant didn't realize he was a fake until the very end.