The story concerns vice squad Detective Sergeant John Dark (Goss), whose partner and mentor Detective Chief Inspector Mortimer Shade (Howarth) is believed murdered by an unknown assailant. However, just as Dark begins to run the killer down, Mort starts making a remarkably rapid recovery. When a series of bizarre underworld killings occur in which Dark and Mort’s major adversaries are taken out by mysterious bestial attacks, it becomes clear that Mort has transformed into something strange and otherworldly.
