Scientist Harry Harrison has a curious obsession: the regeneration of human body parts. And his experimentation comes in handy after his own arm is severed in an accident and he's able to replace it. But things quickly go awry for Harry when the new limb, created from a combination of genes from a lizard and a serial killer, develops a capacity to act on its own. With his mother out to profit from the abnormality, Harry hides underground and even meets a new girlfriend in this netherworld. But when she's kidnapped, Harry puts his mutation to use and sets out for retribution.