Publisher Valve's sleeper hit of 2007 returns with a new single-player story mode, two-player cooperative action, and new physics-based challenges to overcome. As in the original game, Portal 2 involves figuring out a way through a series of rooms within Aperture Science Labs, a futuristic facility that exists within the Half-Life universe. You'll fire a portal gun to create openings and exits within each area, allowing you to traverse the environment and manipulate objects in unusual ways. The GLaDOS computer once again serves as your droll guide through the game, as you navigate the hazard-filled test chambers and encounter new characters along the way. The cooperative mode is notable for featuring a distinct campaign, separate playable characters, completely different test chambers, and reworked puzzles that rely on teamwork.