The "Portal" series, which takes place in the same universe as the "Half-Life" games, is a series of puzzle games developed by Valve. The first game in the series, "Portal", was initially released alongside "Episode Two" in "The Orange Box" on October 10, 2007. The player controls a test subject named Chell as she moves through the laboratories of Black Mesa's primary rival, Aperture Science, completing various tests with a device that allows her to create linked portals in physical space. In the later stages of the game, the player battles GLaDOS, a corrupt artificial intelligence computer that monitors her every move. The game is the spiritual successor to "Narbacular Drop", with many of the same team members working on the game. "Portal" has been acclaimed as one of the most original games in 2007, receiving praise for its unique gameplay and darkly humorous story. An Xbox Live Arcade expansion was released on October 22, 2008, and its sequel, "Portal 2", was released on April 19, 2011.