Players assume the role of Mr. Diaz, a synthetic human hybrid created by aliens. In the game's introduction, he turns on his masters when he kills a synthetic human intended to replace the President's bodyguard, Johnny Slater. Diaz finds himself in a massive underground installation created by the aliens under Manhattan. As the game progresses, it is revealed that the player is actually assuming the role of Slater, who was disguised as Diaz by the Gargatuans. The Gargatuans are an alien race around three feet tall who, after being betrayed by a member of their species who awoke from hypersleep and piloted the ship to Earth, are forced to help said traitor with his genetic experiments. The alien creates clones and hybrids (a genetic mix of human and Gargatuan DNA, resulting in extra-powerful creatures) and intends to conquer the earth through a replacement of its leaders, beginning with the United States. A few Gargatuans have escaped the traitor, and conduct an underground resistance in the woodwork. They found Johnny after he had been cloned and disguised him as Diaz, who they incapacitated and kept unconscious. Johnny regains his memories, which were blocked while he was disguised. The player then must travel even further down the bunker in the hopes of stopping the aliens from replacing the president with a clone and by request of the Gargatuans to defeat the traitor. Johnny's personal motive to help him stay focused is that he must make it back in time to meet his girlfriend under the Christmas tree on the White House lawn. Enemies included clones (unremarkable creations, created mainly for menial labor), agents (resemble the public perception of the secret service, men in black suits with sunglasses), mutants (genetic experiments that resulted in vicious creatures, presumably for military), robots (mostly humanoid, but some were straight-out mechs), and hybrids. One hybrid, created to replace the Secretary of Defense (or possibly State), is Johnny's antagonist for much of the game, before a final showdown wherein the alien creature explains much of the plot.