- Main article: List of mutants in The House of the Dead: Overkill
At the height of the Cold War, Formula X was developed by military scientists to transform ordinary humans into powerful super-soldiers; the project, named Operation Overkill, failed. Samples of Formula X were locked in an underground bunker, which was under a prison run by warden Clement Darling.
In 1991, Clement discovered the bunker. With help from crime lord Papa Caesar and disabled scientist Jasper Guns, Clement sought using Formula X to cure his ailing mother. For unknown reasons, the mutagen was leaked from the facility, infecting the Bayou City populace. After physical and verbal abuse from Caesar (including threats to his sister Varla), Jasper injected himself with Formula X to exact revenge.
After discovering Caesar's link to Formula X, then-rookie AMS agent G and Detective Isaac Washington investigated the mutant threat. They followed Caesar to Clement's prison; Clement executed Caesar and claimed responsibility for the outbreak. In the bunker, Clement transplanted his mother's brain into Varla's body with Formula X; his mother mutated into a large creature, which "G" and Isaac defeated. The Darlings perished when Clement's dead man switch detonated the bunker.