On what appears to be a routine recon mission, Daniel touches an artifact known as the Quantum Mirror and ends up in an alternate universe version where he never joined the Stargate Program, Sam is a civilian, Teal'c is still Apophis' First Prime since alternate!SG-1 never went to Chulak, Jack is a giant douchebag who obviously didn't undergo the character development he went through in The Movie, Abydos and eventually Chulak are destroyed, and the 'SGC' is the 'SGA'.