Agent Sydney Bristow has always held her own in the traditionally male-dominated world of espionage. In fact, she's grown comfortable in her well-earned role as a respected go-to girl. But with the recent addition to APO of her half-sister Nadia, dynamics inevitably shift as Sydney finds herself with a new sibling, roommate, and - even if unintentionally - with competition. Temporarily sidelined after a botched mission, Sydney has no choice but to watch Nadia take over her field role next to Vaughn. But there is little time for insecurities. Their covers blown, Vaughn and Nadia need Sydney's help, and only the unique combination of Sydney and Nadia can help them all survive.