John Voorman, guilty of vehicular homicide, makes a deal with the devil to atone for his crime. In exchange for the ability to cure fatal diseases in children, he agrees to give up his mortal soul. But, as they say, the devil is in the details. John discovers that a demonic, parasitic slug inserted into his body will perform the cures. As John soon learns, its benign healing powers are paired with an even greater desire to kill.