Zombies (referred to as "Mutants" in The House of the Dead: Overkill), are the main enemies of the House of the Dead franchise. They are depicted as corpses that have been reanimated through scientific means.
- Main article: List of Zombies
All zombies vary in appearance and attacking methods. Some lumber towards prey for a bite or arm slash, but most utilize terrain, weapons, armor, and superhuman capacities like enhanced strength and agility. Unlike traditional zombies, those in the House of the Dead franchise are biologically-engineered to kill humans, rather than eat them. However, some zombies (as shown in The House of the Dead III) feast on some form of food source.
It is established that military scientists developed a mutagen, Formula X, after the Cold War. It was intended for "Operation Overkill", a project to create genetically-enhanced super soldiers. The project failed in an experimental state, but samples of Formula X were left in an abandoned compound beneath a prison; in 1991, warden Clement Darling discovered them, planning to use Formula X to save his ailing mother.
Clement enlisted crime lord Papa Caesar to supply bodies for experimentation and coerce disabled genius Jasper Guns for help. Caesar lost control of the compound, resulting in a mutant outbreak hitting Bayou City, Louisiana. The AMS (which had been investigating Formula X since the 1950s) dispatched the threat. However, in a tape recording given to Detective Isaac Washington, Caesar claimed that Clement's "powerful friends" had access to the compound for an unspecified period of time.
Dr. Roy Curien, a renowned geneticist, began desperately seeking a cure for his terminally-ill son Daniel. His "Bio-Rector" project was funded by businessman Caleb Goldman, the founder of the DBR Corporation. Curien's team of researchers dwindled as his experiments turned inhumane; this, coupled with Curien's obsession with breaking the barrier between life and death, drove him mad. He released his creations upon DBR staff members on December 18th, 1998. AMS agents Thomas Rogan and "G" battled the infestation at Curien's mansion. Curien himself died at the hands of his "masterpiece", The Magician.
Goldman took Curien's research; for the next two years, he produced zombies in his DBR headquarters. Because of man-made disasters plaguing the world (including wars, famines, poverty, and global warming), Goldman sought wiping out most of the human population to conserve the balance of nature.
On February 26th, 2000, he organized an outbreak in Venice, Italy. While AMS agents James Taylor and Gary Stewart fought to his headquarters, Goldman tended to The Emperor, a being intended to rule over mankind. The agents destroyed the Emperor, with Goldman committing suicide after claiming that "a successor" will eventually come.
In 2003, Goldman posthumously staged a second outbreak of zombies. His scheme to activate a nuclear missile launch was foiled by James and new AMS agent Kate Green, but most of humanity had already been decimated. James sacrificed himself to destroy Goldman's final creation, The World.
Sometime after, Kate teamed with G to fight the infestation back to its source; they discovered a laboratory with test chambers and farming pools for zombies. After defeating the Magician, the agents left to battle a new threat; in their absence, a limping man in a business suit entered the Magician's incubation chamber, claiming that "the Wheel of Fate cannot be stopped."
Unbeknownst to many, Curien's body underwent a 19-year resurrection process, intended to become said Wheel of Fate; Daniel, now cleared of his terminal disease, watched the development within the EFI Research Facility. In 2019, he rescued Rogan from zombies (including the giant undead security guard Death). Rogan's daughter, Lisa, and a now retired G fought inside to reunite with him. Learning the truth about Curien, Lisa teamed with Daniel to destroy the Wheel of Fate.
In this spinoff to The House of the Dead, a top-secret government project, "Undead Soldier" (U.D.S.), was to use zombies for military purposes. Among those involved was the father of AMS agent Stick Breitling. After his parents were murdered due to U.D.S., a man named ZED sought revenge, planning to turn all humans into zombies. When finally confronted by Stick, Linda Rotta, and Rikiya Busujima, ZED unleashed the "God of Destruction", a powerful U.D.S. within him. The AMS agents defeat him, saving humanity from extinction.
While the exact procedures of the mutation are unknown, there is evidence revealing how the dead are reanimated: mainly, the zombies are portrayed as specimens kept in test chambers. Since Formula X is still experimental, it is postulated that its victims must be in a state of necrosis.
From there, there are numerous possibilities: zombies can be granted supernatural-like abilities, weapons, enhanced physical traits, or be fused with other living things. Newly-awakened corpses are assumed to be infected by the mutagen in an airborne state, thus will be resurrected after death.