Plot summary
The Daedalus appears in orbit, abandoned, when the team knows it is currently on its way to Earth. Investigating, they discover that it is not the real Daedalus. It is jumping between different realities, powered by a so-called "alternate reality drive". Now the team is up against the clock: they must figure out a way to get back to their own reality before the drive burns out. They must also overcome the daunting discovery that another version of themselves explored the ship and died trying to do the same.
Credits
Adapted from Wikipedia contributors, Stargate Atlantis season 5. HTML formatting and reference markers removed.