Newlyweds Peter (played by David Manners) and Joan Alison (played by Jacqueline Wells) are traveling to a European resort. They are joined by Dr. Vitus Verdegast (played by Bela Lugosi) who is on his way to visit a former Army engineer, Hjalmar Poelzig (played by Boris Karloff). The travelers' car crashes in the desolate Hungarian countryside, and they find shelter in Poelzig's house, which is built over the site of a mined military fort. Verdegast accuses Poelzig of causing a military defeat, leading to fifteen years in prison and the deaths of ten thousand men. He also demands information about the whereabouts of his wife and daughter, who were abducted by Poelzig. In a subterranean chamber, Verdegast discovers the dead body of his wife, preserved in a glass case.
Refusing to believe that his wife and daughter died of natural causes, Verdegast tries to shoot Poelzig. But the appearance of a black cat completely unnerves him. The newlyweds soon learn that they are Poelzig's prisoners. Verdegast plays a game of chess for Joan's freedom, but loses. Poelzig, the leader of a cult of devil-worshippers, casts Joan as a sacrificial victim in a Black Mass. But Verdegast and his servant, Thamal (played by Harry Cording), rescue her from the ceremonial altar. To his horror, Verdegast discovers the dead body of his daughter, a victim of Poelzig's maniacal rages. Filled with revenge, he ties Poelzig to a rack and skins him alive. Believing Verdegast is about to harm Joan, Peter shoots him. Mortally wounded, Verdegast blows up the entire house after allowing the young couple to escape.