The Assignment: Keiko is possessed by a Pah-wraith.
Morn is eating raw slug liver. Also Rom turns up at the bar and requests a different breakfast - eggs and bacon and crap, which is apparently O'Brien's favourite. Quark is still bemused that Rom's quit his job at the bar in favour of being an engineer, but fails to lure him back.
Meanwhile Bashir has killed Kaiko's bonsai plants. Molly announces that they're in trouble. Bashir scarpers, leaving O'Brien to meet his wife at the airlock. Keiko seems to take the death of her plants quite well, then tells O'Brien that she's not Keiko - she's possessed Keiko's body, and if O'Brien doesn't do what she says, she'll kill Keiko.
So it must be a Tuesday then.
NotKeiko says she wants O'Brien to reconfigure the communication arrays. Back in their quarters, Bashir drops off a replacement plant, and NotKeiko reveals that there's a surprise party for O'Brien that night.
Rom joins the swing shift for a briefing from O'Brien. While working, O'Brien has the computer scan NotKeiko, but nothing turns up. He's also unable to work out a way to render her unconscious faster than the occupying intelligence can kill her.
That night, O'Brien arrives at the party, but has trouble enjoying it, for some strange reason. After breaking a glass, he goes to bandage it, and NotKeiko reveals that the reconfiguration she had him do was just a test - the real work begins tomorrow.
After the party, O'Brien tries to get more information from her to no avail. She goes to bed, and O'Brien tries to access information on the computer about Pah-wraith, which inhabit the cavern Keiko was visiting when she got possessed.
The next morning, NotKeiko gives him instructions. Instead he goes to see Sisko, only for NotKeiko to collapse on the Promenade. Sisko and Odo question O'Brien, but he doesn't say anything. He visits NotKeiko, who tells him to stop playing up and gives him 13 hours to get the work done.
Rom interrupts O'Brien to say that he's finished his work already. O'Brien swears him to secrecy and presses him to work. Dax interrupts O'Brien to have him double-check something - she's found the reconfigurations and assumed it's a saboteur. Sisko's suspicious of the changes and asks O'Brien if he knows anything about anyone on his repair crews. They're interrupted by a call from NotKeiko and Molly to remind him he has to finish the work. O'Brien shops Rom to the others, and Odo takes him in.
Rom will admit to nothing and insists on talking to O'Brien. Rom's worked out that the changes will create a chronoton beam directed at the wormhole, which will kill all the wormhole aliens. They work out that the pah-wraith are banished wormhole aliens. This established, O'Brien returns to work, and is interrupted immediately by Odo who's worked some things out. O'Brien clouts him. Having finished the work, he rendezvous with NotKeiko at a runabout.
They take off for the wormhole, and O'Brien remotely triggers the chronoton beam. Which hits the runabout and zaps NotKeiko. Real Keiko wakes up, and they return to DS9 where O'Brien provides an explanation.
Rom staggers into Quark's bar and says he's been promoted to the day shift.