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.