Season 4 - Episode Guide

Sydney quits the CIA and joins a Black Ops unit, "Authorized Personnel Only," along with Jack, Vaughn, Dixon, Marshall, Nadia and Sloane.

What Sydney found out in Wittenburg may endanger her relationship with Jack.

Sloane says Valta, a computer that will revolutionize info gathering, was stolen from the NSA headquarters by Martin Bishop, who will deliver it to a german terrorist group.

Sydney and Vaughn are sent on a mission by APO to obtain a sample of Ice 5, a weapon that can freeze a person to death.

Posing as a Chechen couple, Sydney and Vaughn infiltrate a training camp run by the October Contingent.

After being infected by a hallucinogen, Sydney starts to lose touch with reality.

Nadia and Sydney go undercover as wealthy, irresponsible young heiresses to uncover the whereabouts of a deadly chemical.

Sloane steps down as director of his task force and gives Jack full authority over the next assignment, which involves Anna Espinosa, Sydney's old foe.

While Nadia is in a coma, Sark is taken into APO to help them infiltrate Cadmus Revolutionary Front and retrieve the bomb.

Dixon enlists Sydney's to help him prove Sloane is double-crossing the CIA and using APO to steal the Blackwell Index, a master blackmail list.

Sydney must protect the life of a civilian she unintentionally put in danger.

Nadia puts the ones she loves in danger when she forgets to tell them about her criminal past.

When a mission does not go well, Sydney is attacked and her life is put in danger. The only one who can save Sydney is Marshall.

Jack agrees to Sydney's request for clearence on files relating to Vaughn's father.

Dixon is shot by a masked gunman. The shooter approaches and takes off his mask, revealing Vaughn.

In Poland, men posing as airport guards abduct Professor Sinclair, a nuclear physicist. She wakes up bound in a dark room and is greeted by a man impersonating Arvin Sloane.

The APO team watches footage of an explosion at an Indonesian embassy in Copenhagen that killed 15 people, the work of terrorists headed by Milos Kradic.

Sydney tells Vaughn that she hasn't seen her father in two days. They go to his apartment and find a notebook with recent records of Jack's vitals.

The Sloane imposter is questioned about the Rambaldi orchid, but first the real Sloane must remember a painful memory.

A tactical group infiltrates the DSR and steals the Rambaldi artifacts. Sophia says goodbye to Nadia and Sydney, telling them that the Lisbon police confirmed that it is safe.

A giant version of the Circumference/Mueller device is found floating over a city in Russia, making the citizens insane. Sydney, Nadia and Jack search for Irina.

Jack, Sydney, Vaughn and Irina parachute into Sovogda, trying to dismantle a giant red ball turning people into zombies.