The alternate reality versions (from "There But For the Grace of God") use the quantum mirror from thatepisode to arrive at a secured building in the top secret Area 51 in "our" universe. In their reality Jack was married to Dr. Carter before his death at the nads of the Go'uld, Teal's was an enemy, Kowalsky never died after being invaded by a symbiote, and Carter is a Major with short hair.
Dr. Carter begins to suffer from temporal distortion caused by travel through the quantum mirror. "Our" Carter determines that her alternate and Kawalsky will die unless they are returned to their alternate reality - unfortunately that's also a death sentence since the Goa'uld have swarmed that universe's SGC. SG-1 must use their present day resources and knowledge to return with their new acquaintances and overthrow the Goa'uld