SG-1 returns from a mission early, accompanied through the stargate by a weird sound and a flash of light. Then, during their debriefing with Gen. Landry, they reveal that they were exploring a different planet than the one they'd been assigned. Moments later, the stargate activates again - and a second SG-1 team steps through. Dr. Lam determines that, genetically, both of the Mitchells, Carters, Daniels, and Teal'cs are exactly who they claim to be. Because the second group arrived on time and from the correct planet, Gen. Landry guesses that they are the real SG-1. Carter then theorizes that the other team comes from an alternate - though very similar - universe. Under questioning from their counterparts, the duplicates explain that their Earth is about to be invaded by the armies of Origin. Unfortunately, because an astronomical fluke - the passage of the stargate's wormhole directly through a black hole - seems to have caused their arrival in this universe, sending them back to join the fight in their own universe won't be easy. B efore the two Carters can find a solution, the stargate activates again, and out steps a third SG-1 team. Within hours, more SG teams from more alternate universes arrive, until more than a dozen SG-1 units are roaming the base. One of the teams even contains alternate-universe versions of two old friends that Carter, Daniel, and Teal'c never expected to see again: Martouf and Dr. Fraiser. Because both died several years ago in this universe, their appearance is a bittersweet shock for everyone who knew them. The troop of Carters now working to solve this astrophysical conundrum soon brainstorms an extremely complex solution that involves - among other details - flying the Prometheus to the black hole and detonating an explosion inside the singularity to seal the multiversal rift that has developed. This will prevent more teams from coming through - but it will also strand the new arrivals permanently on this Earth. While Dr. Frasier, in particular, pleads with Landry to give them time to find a better solution, he reluctantly decides that his universe must come first. He orders his own SG-1 and the first duplicate team to take the Prometheus and carry out the mission.