In a small town bordering a swampy region, unexplained murders and rumors of mysterious happenings surround the swamp-based home of the reclusive but respected Curt Ingston. Ingston uses a wheelchair and has invited to his home the three doctors who were trying to cure him when his paralysis set in. Already in the household are his grim-humored butler Rolf; a lecherous chauffeur, Lawrie; a mannish housekeeper, Miss Judd; an Eastern mystic, Agar Singh; and Ingston's allegedly mentally ill sister, Margaret. Outside, the gate is watched by a shrivelled old hunchback called Torque. Coincident with the arrival of the three male physicians is the appearance of a lady psychiatrist, Dr. Lynn Harper, summoned secretly by Margaret to prove she is not insane and help her secure freedom from the control of Ingston and Miss Judd. She arrives accompanied by a neighbor: mystery-writer Dick Baldwin, who rescued her after her car broke down in the swamp. Neither Ingston nor Miss Judd welcome her presence, but must contend with keeping her overnight until her car can be repaired. Following dinner, at which Ingston's conviction that the three doctors are directly responsible for his current condition becomes evident, the party witnesses an exhibition of materialization of an Egyptian skeleton by Agar Singh. Dr. Harper is forbidden to meet with Margaret. Then, one by one, the doctors are frightfully killed as they prepare for bed. Suspecting Ingston, Dick and Police Captain Beggs confront him in his room, but discover he is not paralyzed but a quadruple amputee. Suspicion then falls on Lawrie, who was last seen driving a murdered ex-employee of the household back to town, but he, too, winds up dead. Ultimately, Dick confronts the killer outside the estate as he menaces Lynn, and discovers it is Ingston after all: by studying under Agar Singh, he has learned how to materialize arms and legs, hands and feet for himself, long enough to accomplish his evil deeds. As Dick struggles with him to the death, Margaret sets fire to the unholy house, committing suicide while taking the malevolent Miss Judd with her. As the house burns to the ground, Dick and Lynn are saved by Agar Singh, when Singh shoots Ingston.
The New York Times gave the film a negative review, calling it "tedious and fantastic". Author and film criticLeonard Maltin awarded the film two and a half out of four stars, calling it an "intriguing grade-B thriller". On his website Fantastic Movie Musings and Ramblings, Dave Sindelar called it one of his favorites among Universal's minor horror films, commending the film's use of sound as being quite effective. Craig Butler from Allmovie wrote, "there's a lot that's wrong with Night Monster -- but there's also a fair amount of pleasure to be had from this admittedly-second tier Universal horror flick, especially for those who can't get enough of this kind of picture." Graeme Clark from The Spinning Image gave the film 6/10 stars, calling it " minor but not unenjoyable chiller". TV Guide awarded the film 2/5 stars, stating that the film was only "Somewhat creepy".
