The compilation contains five games, all powered by Valve’s Source engine. Two of the games included, Half-Life 2 and its first stand-alone expansion, Episode One, were previously released as separate products. Three new games are also contained within the compilation: the second stand-alone expansion, Half-Life 2: Episode Two, the puzzle game Portal, and Team Fortress 2.

Portal 2 follows the player-character Chell’s fate after the end of Portal. Retroactively patched just prior to the sequel’s official announcement, the ending of the first game shows Chell being dragged away from the remains of GLaDOS by an unseen figure with a robotic voice, later identified by writer Erik Wolpaw as the “Party Escort Bot”. Chell is placed in stasis within Aperture Science for a period of many years, during which the facility becomes over-run by decay and nature.