Season 4 of Alias aired from January 5, 2005 to May 25 2005. Unlike all other seasons of the series, the fourth season aired entirely during the latter half of the American television season. The season showed Sydney, Vaughn, Jack, Dixon, and eventually Marshall and Weiss, joining a real SD-6: a black-ops division of the CIA called Authorized Personnel Only, headed up by none other than Arvin Sloane himself.
Season 4 begins where Season 3 ended, with Sydney uncovering a shocking, classified document called "S.A.B. 47 Project." It is explained that the document authorized Jack Bristow to execute Sydney's mother, who had mysteriously placed a contract on Sydney's life. 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 (Authorized Personnel Only, Part 1). 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 colleague Eric Weiss (brought in after having to be rescued by Sydney and Vaughn, who he previously believed to have left the CIA) (The Awful Truth). Sloane's daughter and Sydney's half-sister Nadia Santos also eventually returns to join APO (Authorized Personnel Only, Part 2).
APO undertakes missions in a similar style to what SD-6 supposedly once did; they do missions which the CIA does not want to be involved with or does not want to be tied down with bureaucracy. During the season, an Arvin Sloane impostor, jokingly identified as "Arvin Clone", acquired the technology to implement a Rambaldi-predicted apocalypse (Another Mister Sloane). 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 (Before The Flood).
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 (Before The Flood).
Following on from last season, it is likely that Season 4 takes place in 2005.
Similar in style to the last seasons, each episode is comprised of the team being given an objective and having to complete it, overcoming any obstacles that come along in the way. This style continues throughout most of the season. Between episodes 16-19 a storyline about a fake Arvin Sloane, dubbed "Arvin Clone", is introduced. For the final three episode the team battles Ylena Derevko, the most ruthless Derevko sister, whose obsession with Rambaldi has led her to create a huge Mueller device above a city in Russia.
- Sydney and Vaughn developing their relationship again.
- The team's move to APO.
- The blossoming relationship between Weiss and Nadia.
- Sydney's anger at her father for killing her mother, and her subsequent efforts to keep the truth from Nadia.
- Vaughn's confusion over his father, who it seems may still be alive.
- Jennifer Garner as Sydney Bristow (22/22)
- Michael Vartan as Michael Vaughn (22/22)
- Ron Rifkin as Arvin Sloane (21/22)
- Carl Lumbly as Marcus Dixon (21/22)
- Kevin Weisman as Marshall Flinkman (21/22)
- Mía Maestro as Nadia Santos (21/22)
- Greg Grunberg as Eric Weiss (21/22)
- and Victor Garber as Jack Bristow (22/22)
- Sonia Braga as Elena Derevko (5/22)
- Angela Bassett as Hayden Chase (4/22)
- Nick Jaine as Maneesha Jain (3/22)
- Michael Kenneth Williams as Roberts (3/22)
- Joel Grey as Ned Bolger (3/22)
- David Anders as Julian Sark (2/22)
- Lena Olin as Irina Derevko (2/22)
- Gina Torres as Anna Espinosa (2/22)
- Isabella Rossellini as Katya Derevko (2/22)
- Rob Benedict as Brodien (2/22)
- Rick Yune as Kazu Tamazaki (2/22)
- Vladimir Mashkov as Milos Kradic (2/22)
- MIchael McKean as Atticus Liddell (2/22)
- Tasa Papadakis as Aleksander Petrovich (2/22)
- Andrew Divoff as Lucien Nisard (2/22)
- Oz Perkins as Avian (2/22)
- Amy Irving as Emily Sloane (1/22)
- Angus Scrimm as Calvin McCullough (1/22)
- Amanda Foreman as Carrie Bowman (1/22)
- Ilia Volok as Ushek San'ko (1/22)
|01||Authorized Personnel Only (Part 1)||January 5, 2005|
|Sydney quits the CIA and joins a Black Ops unit called "Authorized Personnel Only" alongside Dixon, Jack, Vaughn and Sloane. Meanwhile, the truth Sydney found out in Wittenberg endangers her relationship with Jack, and eventually she reveals the awful truth to Vaughn.|
|02||Authorized Personnel Only (Part 2)||January 5, 2005|
|Sydney unwillingly teams up with her father to hunt down a modern-day samurai. Meanwhile, Nadia doesn't want to go back to the spy business, but is forced to when Sydney's life is jeopardized. Sydney finds out the reason behind Jack's misdeed, and Nadia makes an unsettling promise.|
|03||The Awful Truth||January 12, 2005|
|Sydney must seduce a murderous arms dealer in order to track down a stolen NSA code-breaker. Meanwhile, Vaughn fears for Weiss's life, and Sydney and Jack argue about telling Nadia the truth about her mother, and that Jack was the one who killed her.|
|04||Ice||January 19, 2005|
|Sydney and Vaughn are sent on a mission by APO to obtain a sample of a new bio-weapon which can literally freeze a person to death. Meanwhile, Nadia asks Jack for the truth about her mother, and Sydney begins to wonder if Vaughn has really come to terms with Lauren's death.|
|05||Welcome To Liberty Village||January 26, 2005|
|Sydney and Vaughn are sent to infiltrate a training camp run by the October Contingent that has been designed to resemble a suburban American neighborhood. Sydney and Vaughn find themselves in a unique position of having to 'learn' how to pose as Americans. But these civics lessons have deadly consequences for the losers.|
|06||Nocturne||February 9, 2005|
|After being infected with a hallucinogen, Sydney starts to lose touch with reality. Vaughn and Jack must rush to find an antidote before Sydney puts her life - and others next to her - into danger. Dixon and Sloane butt heads over sending Sydney into the field.|
|07||Detente||February 16, 2005|
|While Sydney andNadia argue about Nadia's acceptance of Sloane's fatherly concerns, Sloane worries about Nadia's well-being when she and Sydney go undercover as wealthy, irresponsible young heiresses to uncover the whereabouts of a deadly chemical.|
|08||Echoes||February 23, 2005|
|Sloane temporarily steps down as Director for the latest assignment which involves Anna Espinosa, Sydney's old foe. However, Anna turns the tables on APO when she captures Nadia and forces Sydney to do her biddings. Sydney tells Nadia about the Rambaldi's prophecy which dictates that two sisters will fight and only one will survive.|
|09||A Man Of His Word||March 2, 2005|
|While Nadia is in a coma, Sark is taken into APO to help them infiltrate Cadmus Revolutionary Front and retrieve the bomb, but in return he demands to see Lauren's corpse. Nadia may be the only person with the intel they need to intercept the bomb, so Jack is forced to prematurely wake Nadia from her coma, endangering her life, which doesn't please Sloane when he finds out.|
|10||The Index||March 9, 2005|
|Dixon enlists Sydney to help him prove that Sloane is double-crossing the CIA and using APO to steel the Blackwell Index, a master blackmail list, and to reconstruct the Alliance. Meanwhile, Vaughn discovers something disturbing about his father when he visits his comatose uncle.|
|11||The Road Home||March 16, 2005|
|Jack kills an old friend who is poised to receive a biometrically targeted weapon. Sydney confronts the weapon and saves an innocent civilian, whose live she initially endangers. Vaughn uncovers more disturbing stories of his father, who was possibly a traitor.|
|12||The Orphan||March 23, 2005|
|Nadia puts her own life - as well Sydney's and Weiss's - in danger after failing to unveil her past through a dangerous criminal. Meanwhile, Vaughn finds a person who is somehow connected to his father and Nadia as well.|
|13||Tuesday||March 30, 2005|
|When a mission goes wrong, Sydney is attacked and placed in serious danger. Meanwhile, Dixon is exposed to a deadly pathogen released inside APO which is subsequently placed in lockdown. Marshall is the only one outside of APO able to go and save Sydney, assuming he can get to her before its too late.|
|14||Nightingale||April 6, 2005|
|Sydney and Vaughn risk their lives trying to find out the truth about Vaughn's father. Jack and Sloane's conspiring together against Sydney and Vaughn is revealed. Sydney and Jack travel to the Nightingale facility to shut it down, but Sydney becomes trapped in the room with the Nightingale device. Jack goes to extreme lengths to save her.|
|15||Pandora||April 13, 2005|
|In an attempt to talk to Sydney, Katya Derevko deliberately poisons herself and is temporally taken out from her cell. She unsettles Sydney by telling her that someone framed her mother as a means of getting her killed. Meanwhile, Vaughn becomes a rogue agent by getting involved with the people who might know his father's whereabouts.|
|16||Another Mister Sloane||April 20, 2005|
|Sydney and Vaughn tell Jack about Sloane's alleged involvement in Vaughn's search for his father and Irina's contract on Sydney's life. After cornering Sloane, they both conclude that someone is framing him. The APO team set a trap to catch the fake Sloane, but Nadia fears that this pursuit may revive her father's past obsession – Rambaldi.|
|17||A Clean Conscience||April 27, 2005|
|Nadia's former caretaker Sophia contacts Nadia and asks to meet her. She reveals that someone is trying to track Nadia down, so Sloane and Jack decide to tell their daughters the truth – Ylena Derevko, the third and the cruelest of the Derevko sisters, has been surveilling both Sydney and Nadia for a decade. Meanwhile, Jack's doctor advises him to tell Sydney about his illness.|
|18||Mirage||May 4, 2005|
|Jack's illness causes him to hallucinate about a cure. Sydney must recreate the time when Jack was working for the CIA and hid the only man who might be able to help him now. For this, Sydney must play her own mother. Meanwhile, Sophia moves in with Sydney and Nadia.|
|19||In Dreams...||May 11, 2005|
|The APO team sets a trap in Paris that manages to snare the Sloane impostor. After Jack uncovers the origin of the Sloane clone, Marshall devises a way to shock him back into his own reality. But this technique poses a potential danger to the real Sloane.|
|20||The Descent||May 18, 2005|
|After a tactical group infiltrates the DSR and steals Rambaldi artifacts, CIA director Hayden Chase discovers that the alarms at the DSR were disabled from Nadia's computer, allowing the raid to happen, and shows up at the apartment to question her. Soon Sloane and Jack arrive and suspect that Sophia, who had access to the computer while she was living with Syd and Nadia, is Ylena Derevko.|
|21||Search And Rescue||May 18, 2005|
|Rambaldi's catastrophic endgame begins to fall into place as a giant version of the Mueller device is found floating over a city in Russia, driving the citizens mad. Sydney and Nadia discover Irina, whose death was faked, had been working to counter her sister's organization and prevent the Rambaldi endgame. They all work together to try to stop it.|
|22||Before The Flood||May 25, 2005|
|Jack, Sydney, Irina, Vaughn, and Nadia parachute into Sovogda. At APO, Marshall and Weiss use the Blackwell Index to blackmail the Russian minister into giving them the access codes to a Russian satellite. They learn that the Rambaldi device is broadcasting a frequency that physiologically alters people who drink it and it is irreversible. Elena learns of APO's arrival.|
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