Half-Life: Decay is the the third expansion pack for the award-winning game, Half-Life. It was included as part of the PS2 version of Half-Life. Decay returns to the setting and timeline of the original story, albeit portraying the story from the viewpoint of a different set of protagonists: two scientists working in the Black Mesa Research Facility. Decay is a cooperative multiplayer game, designed to be played by two people working together to pass through the game's levels.