An elite military unit (SG-1) must flee back to Earth when two members of their team inexplicably disappear into thin air. But they return to a world in which the Stargate -- an Ancient device which allows them to travel to hundreds of planets -- was never discovered, and their own history has been erased. Now they must convince a dubious government and military that the timeline was altered by a powerful Alien enemy (Ba''al) in order to conquer Earth, and time is not on their side.