While the DLC pack won't expand Chell's single-player adventure, it will add "Peer Review," a whole new cooperative campaign for robot pals P-Body and Atlas. Details are scarce about the new mission, except that players will "once again match wits with GLaDOS," the nefarious, off-kilter AI. The DLC will also introduce a "Challenge Mode" for both single-player and co-op play. Players will be able to compete for the best scores and then compare their rankings on Leaderboards.