 Valve planned to produce downloadable content for Portal 2 , beginning with " Peer Review " , released on October 4 , 2011 . The content , which is free regardless of platform , includes a new cooperative campaign which extends the game 's story . A week from the end of the cooperative campaign , GLaDOS prepares Atlas and P @-@ Body to deal with an intruder within Aperture Science — the bird that had previously abducted her as a potato . The content also adds a " challenge mode " similar to that in Portal — players try to complete specific chambers with the shortest time or fewest number of portals used , both which are tracked on overall and friends leaderboards . The challenge modes are available for both single @-@ player and cooperative modes . 
