Jack and Lissa Russell, Topaz and Elaine Marston exit their car and approach the front of Marcosa House, a 19th Century mansion that sits in a sulfurous pit-like depression surrounded by high cliffs. Above the boarded door, a stone gargoyle leers. As the foursome ponder a way in, the planks fly off, and the entry creaks open, locking again once they are inside. Immediately, Topaz is accosted by a shadow that throttles her into unconsciousness.
The others move her to a couch in a room where the fireplace spontaneously ignites, while outside, the ground beneath their vehicle gurgles.
Jack recalls the events leading to their presence in the haunted house: while at the bedside of their critically injured friend Buck Cowan, Jack and Elaine heard him murmur that they can save him by going to Marcosa House. Later, Elaine explained she was part of a paranormal research team that was investigating Marcosa House for a dying millionaire, Jerome Selwyn. Their stay there ended with the deaths of the rest of the team including her husband Steven. Desperate to help Buck, Jack, Lissa, Topaz and Elaine visited Selwyn who again offered two hundred thousand dollars if they can return with some arcane means of revitalizing his cancer-ravaged body. So they proceeded to the mansion in Selwyn's limousine, which at the present sinks into the bog.
Within the house, Steven appears apparently alive, while Jack transforms into the Werewolf, despite the next full moon being three weeks away. "Steven's" head then becomes a skull (with a hole in the forehead), and the man-beast slashes at the apparition. It tosses the Werewolf away, then vanishes.
Jack becomes himself again, but the group hears the wooden footsteps of Belaric Marcosa, the occultist who lived there one hundred years before.
While the distraught Elaine goes to rest in her room, Topaz performs a seance with Jack and Lissa. They are visited by a ghostly peasant claiming to be the spirit of the original werewolf whose curse Jack inherited. As he fades away, he implores them to leave Marcosa House.
That night, Jack dreams of a gargoyle that claims the secrets lie within it. Then he's woken up by Elaine who tells him Topaz is missing. They soon find her in the depths of the mansion, where she says her psi-powers have lead her, and she asks Jack to break through a brick wall. Behind it, they find a headless skeleton chained to the wall. Jack recalls his dream with the gargoyle, and leads Topaz and Elaine toward the front door, but they are stopped by Lissa's scream.
Before they can reach her, the peasant, in solid form now, transforms into the Werewolf and lunges at Jack.