http://www.youtube.com/watch?v=pZs6ewODVqc&feature=related
Season Four: 2005
Season 4 begins where season three ended with Sydney uncovering a shocking, classified document called "S.A.B. 47 Project." It is explained that the document authorizes Jack Bristow to execute Sydney's mother, who had mysteriously placed a contract on Sydney's life (this was apparently something of a retcon to cover for actress Lena Olin's presumed not returning to the series, as the first page refers to Sydney as the "active" subject of a "project" that began 17 April, 1975, a possible reference to Project Christmas, and also setting up Jack as either the real head of, or somehow involved with, the Covenant and/or being a descendant of Rambaldi/Rambaldi himself).
Sydney joins a black ops division of the CIA, patterned after SD-6 and run by her one-time nemesis Arvin Sloane. The new division is dubbed "APO": Authorized Personnel Only. Members of APO (all hand-picked by Sloane) include almost all of the recurring characters from previous seasons, including Jack, Vaughn, Sydney's former partner (and third season CIA director) Marcus Dixon, the computer and technical genius, Marshall Flinkman, and Vaughn's best friend Eric Weiss (brought in after having to be rescued by Sydney and Vaughn, who he previously believed to have left the CIA). Sloane's daughter and Sydney's half-sister Nadia Santos also eventually returns to join APO.
During the season, an Arvin Sloane impostor, jokingly identified as "Arvin Clone," acquired the technology to implement a Rambaldi-predicted apocalypse. Using Omnifam, the real Sloane had polluted the world's drinking water with chemicals that caused feelings of peace and tranquility. However, these feelings can be reversed with the Mueller device. The third Derevko sister, Elena, had built a giant Mueller device in Sovogda, Russia, which drove the residents to insanity. Sydney, Jack, Irina, Nadia, and Vaughn parachute in, destroy the device and kill Elena. But Nadia is injected with the tainted water and driven insane. She battles Sydney until Sloane is forced to shoot his own daughter. Nadia is later put into a coma while a cure is sought and Irina escapes again.
The season concludes with Sydney and Vaughn becoming engaged. On a trip to Santa Barbara, Vaughn confides a shocking secret: his name isn't really Michael Vaughn; their initial meeting wasn't coincidental; and that his allegiance may not be to the CIA. Before he can divulge any more information, another car hits theirs and the season ends.
