Using a highly experimental portal device, you'll once again face off against a lethally inventive, power-mad A.I. named GLaDOS. And this time you won't be alone. Meet an expanded cast of characters as you think your way through dangerous, never-before-seen areas of Aperture Laboratories. Break the laws of spatial physics in ways you never thought possible, with a wider variety of portal puzzles and an expansive story that spans a single player and co-operative game mode.