A new Stargate team embarks on a dangerous mission to a distant galaxy, where they discover a mythical lost city -- and a deadly new enemy.

Nearly a third of Atlantis's residents are infected with a deadly virus that causes hallucinations and death, forcing a lockdown of the city.

With a brief opportunity to send a data transmission to Earth, members of the Atlantis expedition compose video messages home to their loved ones.

With a group of Wraith hive ships on the way to Atlantis, the team searches for a place to flee and a way to destroy the city. McKay leads a team to try and activate the Ancients' orbital weapons platform.

All hope seems lost, until Colonel Everett arrives suddenly from Earth. He is determined to defend the city until the arrival of a ZPM, brought by a new ship.

The Daedalus arrives to defend Atlantis from the Wraith onslaught, but it may not be enough when the team discovers that an even larger armada is on the way.

The Daedalus is infected by a Wraith computer virus, while members of the expedition hope to return to Earth.