The Half-Life series of video games share a science fiction alternate history. All of the games are first-person shooters (with the exception of Portal and Portal 2) on the GoldSource or Source engines, and are linear, narrative, single-player titles. Valve Corporation is the developer, and partly the publisher and distributor, for the signature games of the series. Each of these games features the player as the main protagonist, Gordon Freeman, a theoretical physicist initially employed by the Black Mesa Facility. Two of these games, Half-Life and Half-Life 2, are full length titles, while two more, Half-Life 2: Episode One and Half-Life 2: Episode Two, are shorter, episodic titles. A final episode, Half-Life 2: Episode Three, was originally scheduled to appear in 2007, but now has no public release date. A puzzle-based spin-off series, set in the same fictional universe as the Half-Life games, started with Portal in 2007, followed by a sequel, Portal 2, in 2011.