In the Gamma Quadrant, Dax insists on investigating a planet surrounded by an energy barrier, but the barrier cripples the U.S.S. Defiant. 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. Sisko and his crew must choose between the lives they have always known and the lives of their own descendants.