The movie opens with the airplane circling the Earth, A Universal Picture, logo, and the title and credits. At a busy train station, passengers come and go. A train official examines the papers of Peter Alison (David Manners) and his wife, Joan (Julie Bishop as Jacqueline Wells). Satisfied, he closes the door of their compartment and moves on. The Alisons settle into their comfortable accommodations, but are soon interrupted by the conductor, "Excuse me sir, lady, a terrible mistake has occurred. The half sold space in your compartment to a gentleman. It is terrible." The gentleman in question is Dr. Vitus Werdegast (Bela Lugosi). He agrees to make himself comfortable in the passenger car, but Peter insists he join them in their compartment. Werdegast stares at Joan, then at Peter, and finally up at a precariously placed bag above the Alisons. He jumps to his feet and catches the bag before it can fall on the couple. The three introduce themselves. Werdegast explains he is going to visit an old friend. The train rolls on to the first stop where it is pouring rain. Werdegast explains to Peter he was incarcerated at a prison in Omsk on Lake Baikal in Siberia. "Many men have gone there. Few have returned. I have returned. After fifteen years, I have returned!"
The bus from the Hotel Hungaria in Gombos takes the four to the hotel. Werdegast tells the driver to drop him off at the Poelzig house on the way. Werdegast's servant, Thamal (Harry Cording) sits listening to the driver explain the sights. It is still raining heavily and the driver loses control of the bus. It crashes and the driver is killed. The passengers survive, but Joan is slightly injured. Thamal carries Joan to the Poelzig house, while Werdegast and Peter carry the bags. The Majordomo (Egon Brecher) answers the doorbell and lets the weary travelers into the house. On the way to their rooms, Werdegast tells the servant to call the authorities in Visegrad and report the accident and that the driver was killed. The Majordomo informs Hjalmar Poelzig (Boris Karloff as Karloff) by intercom that Werdegast has arrived. Poelzig gets out of bed, puts on a robe and slippers while Werdegast dresses a wound on Joan and gives her a mild sedative, Hyoscine. Poelzig enters the room and Werdegast addresses him, "It has been a long time Hjalmar. The years have been kind to you." Werdegast looks towards Joan and continues, "An accident on the road below. We are all very fortunate to be alive. Mr. and Mrs. Alison are going to Gombos. Mrs. Alison is slightly injured. I took the liberty of bringing them here." Poelzig escorts Werdegast out of the room, leaving Peter to fret over his sleeping wife.
Werdegast confronts Poelzig over his treachery during the war. He does admire the construction of the house, "A masterpiece of construction, built upon the ruins of the masterpiece of destruction." Werdegast reminds Poelzig of his time in prison and then asks about his wife and daughter. Werdegast knows about his wife's travels with Poelzig and traced them to this house. He asks point blank, "Where is she?" Before Poelzig can answer, Peter wanders into the room. Poelzig invites Peter to join them in a drink. Peter explains that he is a mystery writer. When Werdegast sees a black cat, he drops his glass and throws a knife at the creature, killing it instantly. Joan wanders into the room in a drug induced trance. She asks Werdegast, "You are frightened, doctor?" Poelzig explains to Joan, "You must be indulgent of Dr. Werdegast's weakness. He is the unfortunate victim of one of the commoner phobias, but in an extreme form. He has an intense and all-consuming horror of cats." Peter kisses his wife then carries Joan back to her room and puts her to bed. Poelzig escorts Peter and Dr. Werdegast to their rooms. They discuss the origins of black cat superstitions. Werdegast and Peter decide to trade rooms so Peter can be closer to his wife. He asks the doctor if his wife will be well enough to travel to Gombos in the morning and is told yes.
Poelzig wanders the lab petting his cat. He examines a couple of embalmed women in display cases. He returns to Dr. Werdegast's room and is surprised to find Peter. Werdegast invites Poelzig into his room and closes the door that adjoins Peters room. Again Werdegast asks where his wife is. Poelzig replies, "Very well, Vitas. I shall take you to her." The two walk downstairs to the main room. Next they walk down another set of stairs that served as the passage to the gun turrets when the property was a fort. They walk until they arrive at the old chart room. Poelzig turns on a light that illuminates a woman hanging in a display case. "You see Vitas, I have cared for her tenderly and well. You will find her almost as beautiful as when you last saw her. She died two years after the war." When Werdegast asks how, he is told of pneumonia. Poelzig tells the doctor that his daughter is also dead. Werdegast does not believe Poelzig and pulls a gun on the architect. Before he can pull the trigger, a black cat appears and Werdegast falls back against a glass chart on the wall. Poelzig escorts Werdegast back upstairs and invites him to play another game when the Alisons leave.
Poelzig returns to his room and goes back to bed. His wife inquires about the commotion below stairs. He tells Karen (Lucille Lund) to stay in her room the next day. We learn that Karen is Dr. Werdegast's daughter. In his room, Werdegast tells his servant, Thamal, to put away the knife. They must choose their time carefully. The house is undermined with dynamite, so they must be very careful. Poelzig reads his book on Satanism before turning the light out. The passage refers to a ceremony involving a human sacrifice of a woman. The next morning Werdegast looks in on his patient, Joan. She starts to remember the accident, but not the injury or treatment. Poelzig enters the room to check on his guest. Werdegast and Poelzig sit at a chess set and agree to play a game for the freedom of the girl, Joan. Werdegast is invited to witness the ceremony Poelzig plans for the evening, with Joan as the guest of honor in the ritual.
Peter and Joan decide they don't like the house or Poelzig and they want to leave immediately. Two policemen arrive at the house. The Lieutenant (Albert Conti) and the Sergeant (Henry Armetta) question the three men about the accident on the road. Peter explains they were on their way to Gombos when the bus crashed. Part of the road gave way due to the rain and the bus tipped over. Peter explains that he needs to get to Visegrad so his wife can contact her parents in Vienna. All his attempts to depart are thwarted by Poelzig: inoperative car, dead telephone to call for a car. Werdegast and Poelzig continue their game of chess and Poelzig wins. Peter gets his wife and tells her they are leaving, even if they have to walk. Peter and Joan walk to the front door where Thamal stands guard. Peter commands Thamal, "Open the door." Thamal grabs Peter by the throat and knocks him to the floor. The Majordomo catches the fainting Joan. Thamal carries Joan back upstairs and tosses her on her bed. Poelzig enters her room and locks the side door, and then exits and locks the main bedroom door. As Thamal picks Peter up, Werdegast warns, "I hope you won't carry this too far, Hjalmar." The two servants take Peter down into the basement and lock him in a room with a revolving wall.
Poelzig sits at an organ and plays the Tocatta in D Minor by Bach. Werdegast secrets a key into his hand and enters Joan's locked room upstairs. He assures Joan she has nothing to fear from him. He tells Joan, "Please, Child. Listen to me. We're all in danger. Poelzig is a mad beast. I know. I know, I've seen the proof. He took Karen, my wife, murdered her and murdered my child." He tells her he shall have his revenge and very soon. Werdegast tells Joan that Poelzig is a Satanist, and she will play a part in the ritual this evening. The organ music stops and Werdegast knows he must get back downstairs to Poelzig. In parting, Werdegast tells Joan, "Be brave. It is your only chance." Werdegast walks back downstairs and Poelzig motions for the key. A black cat runs into Joan's room. A woman in a long, black dress, with long blonde hair enters the room. Karen introduces herself as Mrs. Poelzig. Joan makes the connection and asks if she is Karen Werdegast. "Yes, yes how did you know my name?" When Joan explains she knows Karen's father, Karen corrects her, "Oh, no, you are mistaken. My father died in prison. Herr Poelzig married my mother. She died when I was very young." Poelzig is outside Joan's door and overhears the exchange, including Joan's news that her father is alive and in the house looking for her. Poelzig enters the room and picks up the cat. His wife scurries back to her room. The door slams shut and we hear the exchange between husband and disobedient wife. Later we will learn he killed her.
It is late evening and Poelzig is outside. It is cloudy and windy. The servants prepare for the guests and the ceremony. Poelzig, dressed in ceremonial black robe, descends the stairs to greet his Satan worshipping accolades. They are dressed formally and stand before a double-X-shaped cross on a dais. Poelzig takes his place behind the cross. Werdegast joins the worshippers, who now don black robes of their own. An organ is played in the background. Poelzig begins chanting in Latin. Joan is dragged from her room by two women and two men. She pleads to be let go. She faints on the altar and slumps over the cross.
Down in the basement, Peter comes to and hears the organ music. He finds the light switch and operates the moving wall. Poelzig approaches Joan. A woman in a white dress turns to observe Poelzig and screams and faints. The distraction allows Werdegast and Thamal to rescue Joan from the altar. Peter finds the correct door and escapes his confinement. He sees one of the embalmed women, hanging in the display case. Werdegast leads the way down the spiral staircase, while Thamal carries Joan. As the Majordomo enters the lab, Peter waits to ambush the man. The two struggle, but the Majordomo succeeds in escaping. He pulls his gun and shoots Thamal. Thamal is strong and kills the Majordomo. As Werdegast tries to escort Joan to safety, she tells him of Karen, his daughter. She explains that Karen is Poelzig's wife. Werdegast finds Karen's body under a sheet in the lab. He screams. Joan and Poelzig enter the room. The two men struggle on the floor. With his last bit of strength, Thamal comes to the aid of Werdegast. He first locks the door. We see blood running from his mouth. Next he grabs Poelzig and the two men carry a struggling Poelzig over to a restraint device. They secure the man's wrists high above his head. Werdegast strips the shirt off Poelzig's body. Poelzig hangs helpless on the very device he so often used on his victims. With a glee in his voice, Werdegast tells Poelzig, "Do you know what Im going to do to you now? No? Did you ever see an animal skinned, Hjalmar? Hey, hey, hey. That's what I'm going to do to you now. Flail the skin from your body, slowly, bit by bit." He retrieves a scalpel from the table top. Joan watches with horror as Werdegast taunts the struggling man, "How does it feel to hang on your own embalming rack, Hjalmar?" We see, in shadow, Werdegast skinning Hjalmar Poelzig. Peter is drawn to Joan by her screams. The door is locked and Peter implores his wife to get the key. Thamal has it in his clenched, dead hand. Werdegast helps Joan retrieve it, but Peter doesn't know Werdegast is helping Joan. He orders Werdegast to stand back, then shoots the doctor when he fails to comply. Joan opens the door for her husband. After explaining he only wanted to help, Werdegast order the Alisons to leave quickly. With his strength diminishing, Werdegast approaches the self-destruct panel. "It's the red switch, isn't it Hjalmar? The red switch ignites the dynamite." He throws the power lever and tells Poelzig that in five minutes all will be destroyed. Joan and Peter exit the house and explosions are heard. They get a safe distance away from the house as it is destroyed. They stop a passing car on the road.
Safely aboard the train, the conductor punches their tickets from Visegrad to Budapest and returns their passport. Peter picks up a newspaper and sees a review of his novel, Triple Murder. "In Triple Murder, Mr. Alison's latest mystery thriller, he fulfills the promise shown by the Sixty-ninth Crime in the Purple Spot. We feel, however, that Mr. Alison has in a sense overstepped the bounds of the matter of credibility. These things could never, by the furtherest stretch of the imagination, actually happen. We could wish that Mr. Alison would confine himself to the possible, instead of letting his melodramatic imagination run away with him." We close with Peter and Joan looking at each other in amusement and the cast credits.