Sydney must undergo an extensive lie detector test to satisfy Sloane's frantic search for the SD-6 mole. Meanwhile, with nemesis Anna Espinosa following her every move, Sydney seeks out the connection that a device may have to the 500-year-old prophetic Rambaldi sketch. The mystery of Kate Jones deepens as Will continues to investigate Danny's death; and Sydney discovers that a keepsake from her mother Laura contains more disturbing connections into Jack's past alliances.

Vaughn asks Sydney to use her friendship with Sloane's wife, Emily, to gain access to a mysterious Rambaldi book which happens to be under lock and key in a safe at Sloane's home. Meanwhile, Will is kidnapped and told to drop his investigation of SD-6 or risk the lives of his family and friends, including Sydney's.

Sloane begins to question where Noah's true allegiances lie, as passion ignites between Sydney and Noah. Meanwhile, Sydney continues to uncover new information about her mother, which disturbes Jack. A rogue assassin known as the Snowman is dispatched by K-Directorate to kill Khasinau, a.k.a. "The Man." Will and Francie confront Sydney about the mysterious airline ticket stub found in her jacket.

Sydney discovers a terrifying secret when she's sent on a case to track down a new generation of weapons. Meanwhile, Vaughn suspects Jack of setting up Sydney's mother to take a fall. Sloane tells Jack the truth about Emily's death.

While Alliance counterintelligence head Ariana Kane tracks Sydney in order to catch her fugitive father, Jack elicits Irina's help in finding Sloane's blackmailer. Meanwhile, Sydney and Vaughn risk exposing their covers to SD-6 when they share a romantic dinner together while on a case in France.

A Rambaldi artifact prophesizes that an apocalyptic event will soon take place, and Dixon's grief leads Vaughn and agency psychologist Dr. Barnett to question his intentions, as his thirst for revenge on Sloane intensifies. Meanwhile Sloane receives a life-changing revelation from a mysterious monk in Nepal, Sydney and Dixon hunt for the killer who stole the heart from his victim.

Sydney and Vaughn are reunited again as a team to prevent Sark from destroying the nation's satellite communications system. Meanwhile, Jack must find a way to stop Marshall from uncovering the identity of a familiar murderer. Sydney accompanies Vaughn and Lauren as they meet with Sloane to ask for a favor.

The identity of the mysterious "St. Aidan" forces Sydney to pull a stunned Will out of witness protection. But the pursuit sends the two old friends on a collision course with Allison -- the woman who ruined both their lives. Meanwhile, a zealous Lindsey arrests Jack and Vaughn as conspirators in Sydney's escape from NSC custody, Sloane, still under pressure from Lindsey, puts a plan in motion to eliminate Sydney, and Dr. Brezzel has an unpleasant run-in with Sark.

Lauren stages a stunning betrayal to steal classified materials from the CIA. Meanwhile, Vaughn comes face to face with his darkest demons in pursuit of his wife; Nadia must decide whether or not she can trust her father, Sloane, in the pursuit of Rambaldi's most priceless artifact; and Sydney receives a shattering revelation that will make her question everything - and everyone - she thought she could trust.

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.

In an attempt to talk to Sydney, Katya Derevko deliberately poisons herself and is temporally taken out from her cell. Once Nadia tells her sister about this, Sydney visits her aunt to tell her to stay away from Nadia. However, Katya unsettles Sydney by telling her that someone framed her mother as a means of getting her killed. Sydney sets off in search of the truth behind this story. Meanwhile, Vaughn becomes a rogue agent by getting involved with the people who might know his father's whereabouts. In the end, he and Sydney find out that the man behind these two plots is no other than A. Sloane. However, things are not exactly as they seem to be.

Peyton tells Sloane that she has made a deal for the Rambaldi Sphere with an ally in China. Sloane arranges for stolen missiles to be sent to Hong Kong. Sloane's plan is revealed to the APO members, who race to stop him before he can use the Rambaldi Sphere to put the plan in motion. But this all leads to a long awaited confrontation between Sydney and Irina.