Sequel to one of the main Valve titles continues the story of Chell, the test subject of Aperture Science. As you will be finding your way through newly constructed test rooms you will be not only supervised by GLaDOS but now accompanied through your tasks by Wheatley, AI core trying to assist you, as you will go deeper into the past of Aperture Science.
A significant change from the previous game is that now you will not be alone during the game, be it the single player story with various characters interacting with you, or a multiplayer cooperative mode, where you can complete puzzles with your friend. In co-op mode, you and another player will be taking roles of two bots, constructed for the sole purpose of completing tests.
Game added mechanics of Gel, covering surfaces of the rooms. Said gel, when placed, can speed you up, make you jump higher and even create more places to set up your portals.