In the Gamma Quadrant, Dax insists on investigating a planet surrounded by an energy barrier. But the barrier cripples the Defiant and causes Kira to be "duplicated" for a moment — she will die without eventual medical attention back on Deep Space Nine. A colony lives on the planet, and the leader, Miranda O'Brien, reveals that in two days, when the Defiant departs, the barrier will cause the ship to crash on the planet — 200 years in the past. Stranded for life, the crew will found this colony. The planet, Gaia, is populated by their descendants! Sisko realizes that this foreknowledge will allow his crew to now avoid that crash, but by altering history, the colony will cease to exist. However, Dax's descendant, Yedrin, has a plan to duplicate the Defiant when it reenters the barrier, allowing the original crew to return home while the duplicate crew can fulfill its destiny by crashing on the planet.
Odo's older self is still alive after 200 years, and looks more human. He reveals to a stunned Kira that he has always been in love with her. Meanwhile, Dax learns Yedrin's real plan is to recreate the crash, ensuring that his people do survive.
Yedrin admits that, as the current host of the Dax symbiont, he feels responsible to protect the society Jadzia's judgment error helped to create. Sisko sympathizes, but will not sacrifice Kira's life or ask his crew to leave behind their own lives just to guarantee the continued existence of their descendants. However, Kira feels they can't run away from their destiny — even if that means she has to die.
Sisko is still unwilling to passively let history repeat itself. He and his sad crew join their descendants for a planting ritual on what will be their final day of existence. O'Brien becomes deeply moved by their plight. Unable to bear the thought of them vanishing, he and the others realize they can't let these people die.
The older Odo tries to convince Kira to not sacrifice herself, but Sisko's people prepare to recreate the crash and face their destiny. However, at the crucial moment, the Defiant veers off course — someone changed the flight plan. They safely exit the barrier, and their descendants vanish as if they never existed. Later, the "real" Odo reveals to Kira that it was the older Odo who sabotaged the ship — sacrificing the colony just to give Kira and his younger self a chance at love. After this shocking revelation, the old friends wonder where their relationship will go next.