The starship Defiant is caught in a type of "quantum-bubble" surrounding an inhabited planet. After investigation, it is discovered that the inhabitants are the descendants of the Defiant's crew. Generations of descendants have been living on the planet for many years and their own history began when the Defiant tried to escape from the bubble. Instead of passing through its energy barrier, the ship was thrown back in time two centuries and crashed with no hope of contacting the Federation or repairing the ship. Faced with that reality, the small stranded crew of the Defiant (including Dax, Sisko, O'Brien, etc.) decided to remain and establish a society.
Unfortunately, the stranded Major Kira died a few weeks after the crash due to neural damage she incurred on the bridge when the ship was passing through the energy barrier at first (entering the bubble). Doctor Bashir did not have the required medical equipment on the Defiant to treat her.
The society presented to the crew of the Defiant includes mostly the descendants of the original inhabitants. The only original "colonists" are Odo, whose Changeling physiology gives him greater longevity than the rest, and the Trill symbiont Dax, in a new host, Yedrin Dax, descendant of Worf and Jadzia Dax.
The crew is forced to choose between repairing the Defiant and escaping the situation (wiping out their descendants and everything they've built), and remaining, allowing the Defiant to crash and letting things evolve as they have been presented to them. After much discussion, they choose the latter. Unbeknownst to them, however, the future Odo, wishing to prevent Kira from dying, changed their flight plan. Instead of flying through the spot in the atmosphere that will cause them to be flung back in time, they escape from the bubble, and wipe out their descendants and the colony they had built.