A young couple is murdered in a dark lover’s lane by a monster with red tentacles. Nearby, at Blackwood Sanitarium, a modern castle in the desert, Dr. Arthur Blackwood goes downstairs to the dungeon and tells Carl, his servant, to get rid of the recently dead woman shackled to the wall. Carl has only a small box, so he chops the victim into pieces with a hatchet, stacks her body parts into the box, and dumps it into a cave beneath the dungeon. Meanwhile, Sheriff Taylor visits the doctor’s office to report the murder and mutilation of the couple a half mile away. He wants to talk to the patients, but Dr. Blackwood assures him that the sanitarium is a rest home, not a lunatic asylum, and his patients are unavailable. He introduces the sheriff to his sister, Melody Blackwood, an adult woman with the mentality of a twelve-year-old, who was institutionalized for violence but is now harmless. Disappointed, the sheriff leaves and assigns two deputies to stand guard outside the castle. Dr. Blackwood asks his sister if she went to the caves the night before and “let her friend out,” and she confesses that she did. Later, the doctor’s wife, Diane Blackwood, whom he married in New York City, arrives at the castle for the first time, driven from the airport by Ernest, the doctor’s leering factotum. After Diane and her husband embrace, she meets Melody, her sister-in-law. Meanwhile, in the dungeon, when one of the female prisoners scratches Carl’s face, he shackles and whips her until Dr. Blackwood’s nurse, Eleanor, stops him. That night, Ernest menaces Diane by walking into her room and turning off the lights. She runs to her husband to complain. In another part of the castle, Melody prepares to visit her “friend,” with her brother’s blessings. That night, the monster kills the two deputies outside the sanitarium, and Sheriff Taylor storms into Dr. Blackwood’s study the next morning to confront him. In the dungeon, one patient, Mr. Zolack, grabs another patient, an attractive redhead, and tortures her. When the doctor later discovers them, he puts the woman in a box with a poisonous snake as punishment for using her body to tempt Mr. Zolack, and then he chops off Mr. Zolack’s head with a guillotine. Later, to punish Ernest for scaring his wife, Dr. Blackwood burns out his left eye with a hot poker. No longer able to tolerate the strange doings, Nurse Eleanor slips away from the castle, but the monster chases her through the woods and kills her. The next day, while the doctor takes several patients to the state hospital, Diane learns from Melody about her friend under the castle. The young woman guides Diane to a secret door in the library that goes to the castle’s underground complex. Below, in the doctor’s laboratory, Diane reads his notes for his diabolical experiments, then stumbles into the dungeon and sees three prisoners being held in cells, next to wall shackles and the guillotine. When the doctor returns that night and finds that Diane has gone through the secret door, he explains his mission and begs her to understand. All men are essentially pure, he says, but they have “a sense of evil,” and the only way to make them good is to remove the evil. With his experiments, he has isolated evil, crystallizing it in a tangible form in which it can be trapped. Diane calls the sheriff and tells him her husband is an insane murderer. Enraged, Dr. Blackwood drags her into the dungeon, but while he is distracted, she escapes into the cave and is nearly killed by the monster. Escaping through an outside entrance, she flags down Sheriff Taylor and his deputies’ cars. The doctor descends into the cave, but finds only Melody and tells her to leave through the cave’s entrance. He returns to the dungeon and kills everybody with a hatchet, then rigs the guillotine in order to chop off his own head. Outside, the sheriff sets off dynamite, sealing the cave. Later, Diane tells Melody she will take her back to New York with her, but as soon as Melody leaves the room, the monster enters and kills Diane.