Barbara Steele plays Muriel Arrowsmith, a wealthy vixen who can't wait for her mad scientist husband Stephen (Paul Muller) to leave so she can get it on with David (Rik Battaglia), her lover and gardener. What the pair don't know is that Stephen has a spy in the house: the old, decrepit maid Solange (Helga Line), whose face looks not so much wrinkled as it looks like it's covered in dried Elmer's Glue. No sooner are Muriel and David gettin' down to some hanky panky in the greenhouse then Stephen returns to catch them in flagrante. He chains them up in his laboratory and sadisically tortures them, promising that the release of death won't come for a long time: "You don't know how long it takes to die from pain!". Muriel, unafraid, hisses in return, "You can kill my body, but I'll never leave you in peace! Never! NEVER!"
Finally, Stephen sets about electrocuting the lovers...but before they die, Muriel has the last laugh. She assures Stephen that he won't see a penny of her riches- she's left her cast wealth and the castle to her 'barely sane' stepsister Jenny. In your face, crazy husband!
After he's killed them, Stephen gets his mad scientist on- he cuts up their bodies, preserving their hearts in a tank full of liquid. He cremates the remains and uses the ashes as a home-made Miracle Gro on his houseplants. He also gives his faithful helper Solange a transfusion with Muriel's blood. Somehow this restores Solange's youthful appearance, and she and Stephen are at long last able to be lovers. Eww.
Because he's a greedy bastard, Stephen ends up marrying Muriel's stepsister Jenny (also played by Barbara Steele- this time in a blonde wig!) with the hopes that he can push her over the edge into raging insanity. He can then have her locked up, take control of all her money, and live happily with Solange.
After he brings her home, Jenny immediately begins having freaky experiences. She hears heartbeats, laughter, and sees blood dripping from the weird houseplants. She has trippy, creepy nightmares about the murder in which she assumes the role of Muriel. Is she going crazy?
Because he needs a doctor to stamp "INSANE" on her hand before he can ship her off, Stephen calls in Jenny's psychiatrist, Dr. Joyce (Laurence Clift) to make a diagnosis. This is where Nightmare Castle loses some steam. The spooky dreams are no longer shown, but rather just talked about. Solange and Stephen have alot of arguments and spend their time plotting their next steps. Stephen tries to kill Dr. Joyce with an electrified bathtub, but only succeeds in accidentally killing the poor butler.
Things pick up again for a fast and furious finale, however. Solange's original transfusion is starting to wear off and it's making her sick...she says her blood is cold and heavy and painful, like mercury. Stephen hooks both her and Jenny up to the Tranfuse-O-Meter (unfortunately, that's just my name for it), killing two birds with one stone: keeping Solange young and getting rid of Jenny at the same time.
Meanwhile, Dr. Joyce finds the tank containing the hearts of David and Muriel. As he sets the tank on a table the lovers return, all grossified and ready for action in a sequence that reminded me of the "Something to Tide You Over" sequence from Creepshow.
Muriel, now half-lovely and half-grody, seeks her vengeance on Stephen. She states her intention to treat him to what she calls the "torment to ecstacy", which apparently just means she's gonna set him on fire. Bye bye, mad scientist!
It's up to David to finish off Solange, which he does quite easily: he unhooks her from the Tranfuse-O-Meter, which...somehow...speeds up the aging process, turning her in to a skeleton right there on the table!
Dr. Joyce rushes into the lab, grabs Jenny, and chucks the disembodied hearts into the burning fireplace. The ghosts of David and Muriel disappear, and all is well in Nightmare Castle.
The first and last 15 minutes of this flick were great- there was some startling imagery, some creepy sequences, some interesting notions about the nature of life and death, and enough gore to be intriguing without being explicit or gross. The film really suffers in the middle hour or so when the pace slows to a crawl. It's unfortunate, because it keeps Nightmare Castle from being the great old gothic horror movie it could have been. What we have here, folks, is a Tiiffany. I give it 5 out of 10 hearts afire.