Set in Manhattan, the game follows a powerful amnesiac shapeshifter named Alex Mercer who must stop an outbreak of Blacklight, a plague that mutates individuals into hideous violent monsters. During his quest, Alex tries to uncover his past while also coming into conflict with both the US military and a black operations force called Blackwatch. Parallel to the game's storyline is the ability to play the game as a sandbox-style video game giving the player freedom to roam Manhattan.