Escapist: "Portal 2 comes with both a single-player campaign (that's a full three times as long as the original game) and a two-player co-op mode that will let you solve puzzles either online with friends or via splitscreen with good friends. Both modes feature new twists, additional puzzle elements and plenty of the tongue-in-cheek "Science will kill us all" humor that made the original such a gem.
In the single-player campaign, you will play once again as Chell, the mute, orange-jumpsuited lady with bionic feet from the first game. Never mind how that's possible, considering the end of the first game depicted her escape from the self-destructing Aperture Science lab. (Developer Valve "updated" the game last year, adding a new ending in which Chell is dragged back toward the underground science lab immediately after emerging victorious.) Things, it would appear, change. Portal 2 asks that you roll with it."