Episode Description Courtesy of ABC.

Season Premiere Sept. 29 at 9/8c
"The Enemy Walks In"

Sydney's world begins to take an even darker turn when her long-lost mother returns with a dangerous agenda. And Mr. Sark, a wildly ambitious freelancer whose close alliance with "The Man" wreaked havoc on SD-6, emerges as a strong presence.

As for Jack, his life is thrown into turmoil as old wounds are opened with the unexpected re-emergence of his supposedly deceased wife. Complicating matters further is Vaughn's uncertain fate and Dixon's suspicions about Sydney's allegiance.

Within SD-6, Arvin Sloane, now a full partner in the malevolent Alliance of Twelve, finds himself faced with the burdens inherent with his advanced position; and Marshall, SD-6's brilliant yet socially inept gadget maker, seeks to make his greatest creation ever.

Meanwhile Sydney's friend, Will becomes reluctantly involved her world her while roommate Francie, remains precariously unaware of Sydney's true identity.