The Defiant's cruising back to DS9 after a mission. Kira admits she and Shakaar broke up cause the Prophets said they weren't meant to be together. Odo excuses himself. Because, y'know, he fancies Kira and now she's available... Dax discovers a weird energy barrier around a nearby planet and wants to go look. Though everyone's keen to get home, Sisko okays it. They get zapped by the barrier and the ship requires repairs. They're hailed by the human inhabitants, who greet Sisko by name. On beaming down, they meet Miranda O'Brien and Yedrin Dax, who tell them that when they try to leave the planet in a couple of days time, they'll be flung a couple of hundred years back in time, and found the very settlement they're now standing in.
Time travel makes everything fun!
Worf has a reputation, apparently. A computer-generated Quark is teaching the children. Also Kira died a couple of weeks after the crash due to being zapped when they passed through the barrier. Sisko says now they know about the accident, they'll be able to avoid it, however that means that the colony will be retconned. Yedrin thinks that they can solve the problem using a quantum doubling effect. Dax thinks it might work.
Kira is stuck in sick bay with Odo, who's in a container because the barrier's preventing him from shapeshifting. She's met by a a significantly-older Odo who professes his love for her. Though taken aback, she agrees to let him show her around the planet.
Dax says the duplicating plan is going to work. Worf meets the "Klingons", a bunch of people descended from Worf who live as Klingons. Yedrin talks spoilers with Dax. O'Brien is grumpy. No change there then. Kira visits her own grave. She's confused over how this doubling thing affects destiny and impressed by the changes in Odo.
Dax finds something odd in the sensor logs from the crash - the quantum fluctuations have been faked by Yedrin. If they attempted the doubling plan, it would fail and they'd be stranded in the past.
Dun dun duuuuun!
The pair confront Yedrin, who points out that the 8000 people on the planet will disappear if the ship safely returns to the station. Dax responds with the point that Kira will die, and Yedrin confesses that he's responsible for them being there, because Dax insisted that they visit the planet. Sisko's determined to leave the planet. Kira angsts. The population is depressed. The "Klingons" bitch to Worf and ask Worf to kill them so they can have honourable deaths. He says he'll do it... tomorrow.
The next day, Kira visits her grave again. She says she can't let Sisko take the ship back to DS9. She talks to the rest of the Defiant crew, who debate whether they should leave the planet or go back in time. Sisko's intent on going back to the station, end of story.
Back down on the planet, they find the people planting crops and join in. O'Brien reports that the repairs are finished, and gets roped into planting as well, much to Sisko's amusement. Worf turns up with the Klingons and gets them to pitch in as well. O'Brien and Sisko start having second thoughts about retconning the colony. Dax gets the appropriate readings off Yedrin so they can replicate the time warp.
Odo is disappointed that Kira's chosen to die so that the colony could live. The Defiant lifts off, and they send out a probe with personal messages on and then head for the temporal anomoly - but the autopilot veers them away at the last moment, taking the Defiant out of the barrier. The colony has vanished.
Sisko and Dax speculate on who changed the flight plan and think it was Yedrin, unable to face dooming everyone again. Odo tells Kira that old Odo linked with him and he knows everything that happened. Also old Odo was the one who changed the flight plan. Kira is appalled.