In 1972, a Hollywood producer's swinging party takes a strange turn when an uninvited guest comes for more than sex and drugs.

When Detective Mortimer Shade is somehow killed in a freezer, a parasite called a grail possesses his body, revives him, but he needs blood to stay alive. His partner John Dark accepts the new situation and together they become vigilantes, judging and killing the bad guys, with Shade sucking their blood with his claw. However, Dark notes that Shade is losing the rest of his humanity and becoming a monster, being aware and afraid of the danger Shade represents to mankind and trying to stop him.