Children of Time

Passing through an energy barrier around an unexplored planet, the Defiant is hit by a discharge that leaves Kira with serious neural damage. On the surface the crew meet 8,000 people descended from themselves. When the Defiant tries to leave, the barrier will throw it 200 years into the past, where it crashes and the survivors found the colony. In that history Kira died weeks after the crash. Two of the original crew live on: Yedrin Dax, a later host of the Dax symbiont, and a much older Odo. Yedrin proposes a way to duplicate the ship so both futures can exist, but Jadzia finds he faked the figures to make sure the crash happens. After much soul-searching, the crew choose to go back and save the colony. Instead the Defiant escapes safely and the colony vanishes. The older Odo had secretly changed the flight plan, sacrificing everyone to save the woman he loved.