Once Stephen leaves for a scientific conference in Edinburgh, Muriel sits at the piano and plays the stately Mazurka that will become her theme music through the rest of the movie. Evidently, the music is a signal, for the studly gardener, David (Rik Battaglia), comes running to her bedroom when he hears it. Together they run to the greenhouse and sink to the floor in a passionate embrace -- but instead of the elegant Mazurka, the background music for this scene is a sinister organ fugue, warning us that this love scene is bound for an unpleasant conclusion. The camera pans away from the lovers, across the greenhouse, to a figure with a riding crop. Stephen hasn't gone to Edinburgh after all.

Catching the lovers in the act, Stephen whips David unconscious and overpowers Muriel. In the uncensored version, a lengthy and brutal torture scene follows, but so far I've only seen the US print. In my version, Stephen chains his victims to the wall and whips them, but he can't break them. Muriel laughs at him, explaining that she has secretly made a new will, leaving the Castle to her feeble-minded half sister, Jenny. Stephen is taken aback, but his plans are set: he ties Muriel to a bed and drips acid on her, and then electrocutes David on the same bed. He removes their blood and their hearts (which he places on a skewer in a tank of formaldehyde); then he burns the bodies, and places Muriel's ashes in the pot of his favorite plant.

Now, jealousy may have helped Stephen carry out his plan, but it was by no means the main reason. It seems he has a bizarre relationship with the crone Solange. He intends to use the blood of the murdered pair to restore Solange's youth and beauty. There's something unusually disturbing in all this -- Solange is so decrepit that Stephen's obsession with her borders on necrophilia. And even if he does restore her youth, his mistress is still an eighty-year-old woman in a girl's body... and with his wife's blood.

But in order to continue with his experiments, Stephen needs the Castle. And the Castle is due to be inherited by Muriel's half sister. Though Solange despairs at this turn of events, Stephen reassures her. Jenny has a weak mind, and has been institutionalized until very recently. Surely it won't be too much trouble, in a crumbling mansion tenanted by an evil genius, to drive her mad all over again?

The day Jenny arrives at her new Castle, Solange is seen watering The Plant. But it's a young, beautiful Solange! We can only guess how she and Stephen have been celebrating the success of the experiment. However, Solange gets a shock when Jenny emerges from the carriage: she is identical to her late sister, except that her hair is blonde instead of black. Then Solange gets an even bigger shock: Stephen has persuaded Jenny to marry him!

This was not part of the plan, Solange insists furiously when Jenny has gone to unpack. Stephen quiets her, assuring her that it is only a step in making sure the Castle will soon be theirs. In fact, no sooner is Jenny alone for the first time in the Castle when she is frightened by a snake... one of Stephen's animals which has "escaped". So sorry.

Stephen gives Solange a vial containing a mild "halluci-NO-gen", as he puts it, to add to Jenny's wine. Stephen hopes that the drug, combined with the oppressive atmosphere of the old Castle, will hasten Jenny's collapse. Sure enough, on her first night in the Castle, Jenny has a terrifying dream. She hears the sounds of two hearts beating somewhere behind the walls, and when she goes to investigate, a strange wind and ghostly laughter push her back into her room.

Turning back, she sees The Plant -- "Nepenthe irridata," as Stephen crowingly described it -- begin to bleed from its thick, phallic bloom.

As Jenny tries to go back to sleep, the room seems to spin around her. She suddenly finds herself in a coffin. David appears and draws her out of the tomb. As a wordless women's chorus sings the Dies Irae in parallel triads, he leads her to the greenhouse, where everything appears to be glowing. As she sinks to the ground with David, a faceless figure bearing a riding crop appears and strikes David down.

(This scene was supposed to have been shot in slow motion; I've read somewhere that there was a technical reason why they couldn't do this, though I've forgotten the details. Anyway, the actors mime the slow-mo.)

Jenny awakens with her hands around Stephen's throat, shouting "David! David!" Stephen is undismayed by the fact that Jenny has never heard of David. To him, it's either a coincidence or the effects of the Castle on a disturbed mind. Still, he's momentarily unnerved when he discovers that instead of the halluci-NO-gen [sic], Solange had given Jenny "a harmless saccharose" -- sugar water. Stephen recovers his composure, and reassures himself that Jenny is clearly much sicker than he had thought.

The following evening, Jenny sits at the piano and begins to play Muriel's Mazurka. Stephen and Solange, in an adjoining room, are startled when the music suddenly breaks off. They are at first unable to find Jenny, but the girl's scream leads them to the entrance to the family crypt. Jenny claims to have walked there without knowing it, only to find herself locked in. Stephen points out that the door was open when he and Solange arrived.

Stephen, secretly delighted, calls for Jenny's alienist, Dr. Joyce (Lawrence Clift). Dr. Joyce is deeply disturbed by Jenny's deterioration, but quickly begins to experience strange things himself. He and Jenny find an earring in the greenhouse, exactly where Jenny dreamed it would be. Stephen and Solange dismiss this by telling the doctor that Jenny has acquired the habit of hiding things about the house, and then forgetting she has done so.

However, Joyce continues to experience unexplainable and disturbing events. He is drawn to the crypt, where Muriel's tomb flies open -- and is revealed to be empty.

A disturbing event of a different kind occurs one night at dinner. Solange accidentally cuts herself, and both she and Stephen react with strange urgency. Stephen explains that Solange has a rare blood condition, and must be treated immediately. Joyce, perhaps realizing that hemophilia occurs only in males, is perplexed.

Now Stephen is worried. Joyce knows enough to hang him and Solange, if he manages to put the pieces together. Also, there seems to be a very unprofessional sort of relationship growing between Dr. Joyce and his former patient; and while Stephen doesn't have any tender feelings toward Jenny, she is still his wife.

Stephen attempts to get rid of Joyce by electrocuting him in his bath. Unfortunately for him, but fortunately for his intended victim, the butler, who has forgotten to provide clean towels, goes into the bathroom first. The old man (whose entire existence in the film is so that he can be killed in this scene) knocks the soap into the bathwater. As he reaches in to get it, Stephen, hiding behind the wall, thinks the splashing is Joyce getting into the tub. The wrong man gets electrocuted, though Stephen is able to convince Joyce that the butler has died of a heart attack.

Stephen comes up with another plan: he will take Jenny away from the Castle, with its ghosts and terrors, so that she can forget her nightmares and live in peace again. Joyce is obviously disappointed, but agrees that it's a good idea. in the meantime, Stephen attempts to separate Jenny from the doctor still further by suggesting angrily that Joyce is trying to take advantage of her.

(Solange, in the meantime, is becoming ill. She complains to Stephen that Muriel's blood is growing heavy and cold in her veins. She must have new blood, or she will die.)

Dr. Joyce agrees to go back to his clinic, but before he goes, Jenny begins behaving even more strangely. She begins to take on more of her dead sister's personality, and makes obscure, seemingly meaningless statements. She takes particular care to linger over the family crest, a pair of hearts engraved on the base of a statue of two lovers. Joyce leaves, bewildered.

Jenny packs in preparation for her trip. However, Stephen has no intention of letting her go anywhere. He advances on her menacingly, and knocks her unconscious. Carrying her down to his laboratory, he straps her to a table. She regains consciousness briefly, and before Stephen chloroforms her, he takes sadistic joy in informing her of his plans.

In the meantime, Dr. Joyce, who is evidently not as gullible as he seems, has returned and snuck back into the castle. Pondering over Jenny's cryptic, un-Jenny-like statements, he investigates the statue with the family crest. The crest turns out to be a switch mechanism, which opens the base of the statue. Inside, Dr. Joyce finds the two hearts on the skewer. Appalled, he removes the hearts from the skewer...

Suddenly, there is a ghastly laugh. Two pale figures appear in one of the upper windows, then disappear. Joyce looks around, and sees the figures re-appearing in the doorway. He backs away in terror, but as he nears the rear door, Stephen appears behind him and hits him with a candlestick. Then Stephen sees the shapes as well, and the candlestick drops from his nerveless fingers.

It's Muriel. David is with her, his face torn and bloody; but Muriel motions him to attend to matters elsewhere, and he disappears. Muriel advances to Stephen. Her luxurious black hair falls over half her face, obscuring it; but the other half of her face is deathly pale, with deep skull-like shadows under her eye.

While all this is going on, the reanimated David stalks into the laboratory. Grabbing a scalpel, he lunges at the terrified Solange, and cuts open her wrists. As Solange's stolen blood drains from her body, she ages, finally deteriorating into a pile of bones.

Fortunately, Joyce has recovered from the blow to the head, and runs into the lab in time to stop David from killing Jenny, too. The doctor and Jenny run from the lab, with David in hot pursuit. Joyce bars the door to the hall, but their escape is cut off by Muriel. Muriel advances on them as David batters in the door. Joyce, thinking quickly, grabs the hearts from the table and throws them into the fire. Instantly, the spectres vanish, and Joyce and Jenny run off into the stormy night.