Alternate reality versions of Carter and Kawalsky use the quantum mirror 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 hands of the Goa'uld, Teal'c is still in the service of Apophis, Kawalsky is still alive, and Carter is a civilian Doctor.
Dr. Carter begins to suffer from temporal distortion caused by travel through the quantum mirror. "Our" Carter determines that her alternate will die unless she is returned to her 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.