Four convicted murderers - Thaw, Goose, Dyke and Clay - agree to take part in a drug trial in return for their freedom. Each of them, who all protest their innocence, must take a single pill. Soon, Thaw falls desperately ill and dies confessing his true guilt. Dyke dies a few hours later. Goose and Clay remain, and Goose reveals himself as the control specimen. Then Clay discovers that the pill contains a genetically modified worm that feeds on human guilt. Clay protests his innocence, but his only chance of escape is to kill Goose. Then the worm takes over.