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.