Paul Toombes (Vincent Price) is a successful horror actor whose trademark role is Dr. Death, a skull-faced killer. During a party in Hollywood showing off his fifth Dr. Death film, he announces his engagement to Ellen Mason (Julie Crosthwait), who gives him an engraved watch as an engagement gift. Later that evening, however, adult film producer Oliver Quayle (Robert Quarry) reveals Ellen had worked for him previously, in adult films, and had also slept with him. Distraught at Toombes' angry reaction, Ellen returns to her room, where a masked man in dark garb, similar to Dr. Death's attire, approaches her with a knife. An apologetic Toombes comes in shortly after, only for her severed head to fall from her shoulders when he kisses her from behind. Though he is acquitted of the crime, Toombes' career is destroyed as he spends several years in a mental hospital, where even he is not sure whether he killed Ellen or not.
Twelve years later, after his release, Toombes is called to London by his friend, screenwriter Herbert Flay (Peter Cushing), who has partnered with Quayle to produce a Dr. Death television series for the BBC. While on the cruise ship en route to England, Toombes encounters a persistent young actress (Linda Hayden), who steals his watch and follows him through London and eventually to Flay's house. In the spider-infested basement, Toombes discovers Faye Carstairs (Adrienne Corri), the former female lead in one of the Dr. Death movies and now Flay's reluctant wife, driven mad after being disfigured in a car accident. She keeps the spiders as her 'pets'. Outside Flay's house, the young actress discovers the masked and caped figure walking the grounds; believing it to be Toombes, she approaches him, only to be killed with a pitchfork. When her body is discovered, Scotland Yard suspects Toombes, as the killing resembles a scene from one of his films.
Unimpressed to find that Quayle has given Dr Death an 'assistant' for the TV series, since he never had one in the films, Toombes publicly berates his female co-star on set; she is soon found hanged by her hair, another scene from a Dr. Death film. Scotland Yard questions him but finds no conclusive evidence. Toombes is harassed by the parents of the actress from the boat, who have found the watch that was stolen from Toombes. They threaten to deliver the watch to the police unless he pays them a ransom. However, the masked man lures them into the house and impales them both with a broadsword. Faye discovers the bodies and is horrified. On the set, the series director is crushed by a descending bed canopy in a trap intended for Toombes. Later, Toombes is chased through the BBC studio by the masked man while on his way to an interview. Julia Wilson (Natasha Pyne), Quayle's public relations chief, discovers a contract in Quayle's files, but is killed by the masked man; Toombes discovers her body in a replication of the death of Ellen, where Julia's body is seated in front of her dressing table. Toombes relapses into a state of nervous semi-madness. Taking Julia's body onto the set, he locks himself inside, turns the camera on, and sets it ablaze.
Believing Toombes to have died in the fire, Flay signs a contract to take his place as Dr. Death. Later he watches the reel of film from Toombes' studio 'death' in his home – only to see Toombes seemingly walk out of the screen, burned but apparently very much alive. When Toombes demands to know why Flay wishes to destroy him, Flay rages that he had written the Dr. Death role for himself, but was passed over in favour of Toombes; he murdered Ellen to frame Toombes in the hopes of destroying his career but was still not given the role. He then reveals that the contract that Julia had discovered stipulated that if Toombes died, Flay would take over the role of Dr. Death. The two struggle into the basement, where Faye rushes in and stabs Flay in the back. He falls into her pit of spiders and they devour his flesh. Toombes applies makeup to his burn-scarred face, now looking similar to Flay, and sits down to dinner with Faye. Faye says she has made Paul his favourite meal - sour cream and 'red herrings'. They both laugh.
Over the credits, the song "Day is Done" plays. This was actually sung by Vincent Price.
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