Agent Sydney Bristow has always held her own in the traditional boy's club of espionage. In fact, she's grown comfortable in her well-earned role as respected go-to girl. But with the recent addition of her half-sister, Nadia, dynamics inevitably shift as Sydney finds herself with a new partner, roommate, and—even if unintentional—competitor. APO's mission to retrieve a mysterious biological weapon is compromised, and in the melee Weiss is infected. Driven by concern and guilt, Sydney searches for the only known vaccine—until her cover is blown and she must watch Nadia assume her field role next to Vaughn. But when Vaughn and Nadia's cover is blown, only Sydney and Nadia's collective strength can keep Sydney's "family" alive.

The Ghost, a legendary figure who has gone by so many aliases his real identity is a mystery, has created a double-edged hallucinogen that is both a blessing to psychiatry and a powerful weapon that can ignite mass hysteria. When the mad scientist is double-crossed by one of his own minions, Agent Sydney Bristow must accompany him to his undercover clinic in Switzerland and help him learn which of his protégés stole the drug and sold it to a chemical weapons dealer. However, this plan doesn't sit well with Jack Bristow, who shares a past with the Ghost....In fact, he thought he killed the man in a Vietnamese jungle years ago. Now Jack must determine if his mind is playing tricks on him, or if their wartime encounter was just another one of the Ghost's twisted psychological experiments....