THE DEVIL'S NIGHTMARE

Baron von Rhoneberg, a former World War II German general, sacrificed his daughter as the war ended. He did so because his family was placed under a terrible curse; the first-born female of every generation was to become a succubus. Many years later, he tells the story to a reporter who wishes to write an article about it and take pictures of his castle. However, the Baron opposes any photographs being taken. Despite his protests, the young woman goes up to visit the castle and takes pictures but is killed when a dry thunderstorm suddenly rolls in while she is in close proximity to it. Her body is taken back to the town, where it is discovered she has a burn in the shape of a cloven hoof on her arm, which is confirmed as the Mark of the Devil.

Sometime later, a group of tourists become stranded when the bridge they have to cross has been destroyed in a flood. They meet Satan in the guise of a strange-looking man who recommends that they take the ferry boat, but they arrive too late to catch the last ferry of the day. They are then directed to an old castle that offers room and board. When they arrive, one of the doors opens by itself, and a piece of the façade breaks off, nearly killing one of the tourists. Hans, the butler, greets them and shows them to their rooms, giving them a briefing of the history of three of the rooms, one of which bears the same cloven hoof mark on the floor tiles in front of the fireplace. After the guests have been accommodated to their rooms, Hans goes down to a laboratory basement and informs the Baron who is practicing alchemy. Over dinner, the Baron explains his family's history to his guests. His ancestor made a pact and sold his soul to the Devil in exchange for his services. Satan demanded that the eldest daughter of each generation become a succubus. When asked if he ever had a daughter, he shakes his head no.

A young woman named Lisa Müller also comes to stay at the castle and proceeds to seduce each tourist according to their own personal weaknesses, then kills them, using their own sin against them. Each tourist is a representative of one of the Seven Deadly Sins. Matt Ducard represents Gluttony and dies by choking to death while gorging on food and wine. Nancy dies representing Greed by drowning in a hidden treasure hoard of powdered gold. Howard, representing Envy, is killed when he is guillotined, and Corrine, embodying Lust, is murdered when she is trapped inside an iron maiden while they are in the middle of an adulterous tryst. Short-tempered old Mr. Mason represents Wrath and dies when he is thrown out of a window and is impaled on an iron fence below. Regine dies as Sloth when a snake kills her in her sleep. Only the seminarian, Alvin Sorelle as Pride, seems immune to Lisa's seductive charms. He uses his cross to burn her face, causing her to retreat. At that point, Satan appears and tells Lisa that he will take care of Alvin. Satan follows Alvin to the chapel, where Alvin finds sanctuary. The devil says he will wait for him. Alvin offers him a bargain instead: He says Satan can have his soul if he releases the souls of his travelling companions. Satan agrees, and gives Alvin a contract to sign. Using a dagger, he cuts a cross in his wrist and uses the blood to sign the contract, which immediately bursts into flames.

The next morning, Alvin awakes to find that the dead tourists eating breakfast before they set out to continue their trip, as though the previous night's events had never happened. Alvin tells them he had a dream about a succubus. The Baron is then wounded in a fencing accident with Hans, and Alvin waits with him for an ambulance. The Baron confesses to Alvin that he had lied; he did have a daughter and killed her in her cradle. After a conversation with Martha, the housekeeper, Alvin learns that the child the Baron stabbed was not the succubus. Lisa is Martha's daughter from an affair with the Baron's brother, Rudolph von Rhoneberg, and Lisa is the eldest daughter. Alvin dismisses Martha's claims, saying that there is no such thing as a succubus. Alvin chooses to remain at the castle with Lisa while the other tourists go on. However, when he visits the chapel again, he sees the burnt remains of the contract along with the blood-stained dagger. As Alvin and Lisa watch the tour bus heading back to the main road, the bus suddenly swerves to miss a funeral wagon driven by Satan and goes over a cliff, killing everyone aboard. Lisa thrusts herself into Alvin's arms. Lisa and Satan smile at each other as her and Alvin's embrace becomes sensual.

Lisa Müller seduces and kills the tourists according to their sins, but is repelled by the seminarian, Alvin, who uses his cross against her. Satan appears to deal with Alvin himself, leading to the final confrontation in the castle's chapel.

The ending reinforces the film’s morality play structure, with each character’s vice leading to their demise at the hands of the succubus. Alvin’s resistance and Satan's personal intervention highlight the eternal struggle between virtue and evil, with the castle remaining a site of infernal influence. The resolution leaves the curse’s legacy unbroken, giving the story its chilling, ironic sting.