There But for the Grace of God (S01E19)

While exploring an alien stargate complex on P3R-233, a world that appears to have been destroyed by the Goa'uld, Dr. Daniel Jackson discovers a slab that, when activated, turns into a shimmering mirror. He touches the mirror and gets a mild jolt but thinks nothing of it until he returns through the stargate. Emerging from the wormhole, Daniel finds himself in an alternate reality, a place that looks like Earth but where nothing is quite as it was. The most distressing difference is that this world is under attack by the Goa'uld, who have wiped out 1.5 billion people and are about to capture Stargate Command. Col. Jack O'Neill faces off against Teal'c, who in this reality is still loyal to the Goa'uld, and Daniel tries to escape through the stargate with information that might save his world from the fate of this alternate reality — if he survives the jaunt home. (Metro-Goldwyn-Mayer (MGM))