Baron von Rhoneberg (Jean Servais), 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.
Some time later, a group of tourists become lost during a bus trip. They meet Satan in the guise of a strange-looking man (Daniel Emilfork) 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 which 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 (Maurice De Groote) the butler greets them and shows each of 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 (Erika Blanc) 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 (Christian Maillet) represents Gluttony and dies by choking to death while gorging on food and wine. Nancy (Colette Emmanuelle) dies representing Greed by drowning in a hidden treasure hoard of powdered gold. Howard (Lorenzo Terzon), representing Envy, is killed when he is guillotined and Corrine (Ivana Novak), 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 (Lucien Raimbourg) represents Wrath and dies when he is thrown out of a window and is impaled on an iron fence below. Regine (Shirley Corrigan) dies as Sloth when a snake kills her in her sleep. Only the seminarian, Alvin Sorelle (Jacques Monseau) as Pride, seems immune to Lisa's seductive charms. When six of the seven tourists are dead, Satan appears to Alvin. Alvin offers his soul if Satan will return the dead tourists to life, to which Satan agrees. The next morning, Alvin awakes to find that the dead tourists have indeed been returned to life as though the previous night's events never happened and are having breakfast before they set out to continue their trip. Even Alvin himself remembers it as only a dream.
The Baron is wounded that morning in a fencing accident with Hans, and Alvin waits with him for an ambulance. The Baron confesses to Alvin that he 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 that Lisa is the eldest daughter. Alvin dismisses Martha's claims that Lisa is subsequently a succubus. Alvin chooses to remain at the castle with Lisa while the other tourists go on. 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. Alvin enfolds Lisa in his arms. Lisa and Satan smile at each other, knowing they have claimed their souls once again.
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