During a full moon, two Llanwelly grave robbers break into the tomb of the Talbot family, where they inadvertently awaken werewolf Lawrence Stewart Talbot. Later, an injured Larry is taken to Queen's Hospital, where he operated on by Dr. Frank Mannering. He is then questioned by Inspector Owen as to the nature of his skull fracture, of which Larry has no memory. Owen calls the Llanwelly police station, where he is told that Larry has been dead for four years. With the next full moon, Larry once again transforms into a werewolf, breaks out of the hospital and kills an unsuspecting policeman. The next morning, Larry tells Mannering and Owen of his werewolf curse, but is not believed. Mannering tells Owen that Larry is suffering from a brain injury and has become a "lycanthrope," a man who imagines himself to be a wolf. Later, Mannering and Owen travel to Llanwelly, where they visit the Talbot tomb and discover that Larry's body is indeed missing. Mannering then learns that Larry has escaped from the hospital, having bitten through his straightjacket. Larry searches numerous gypsy camps until he finds Maleva, the mother of the werewolf who infected him. She agrees to watch over him and offers to take him to a man who "can help him." They travel across Europe to the village of Vasaria, only to learn that the man, Dr. Frankenstein, has died. As they leave the village, a full moon appears and Larry kills once again. The townspeople rally together to find the murdering "wolf," and chase Larry into the ruins of the Frankenstein castle, where he awakens the Monster from its icy grave. Later, Larry meets with Frankenstein's daughter, Baroness Elsa, pretending that he wishes to purchase the old castle. He asks her for her father's records, but she claims no knowledge of them. That night, the Festival of the New Wine begins, and the mayor asks both Elsa and Larry to attend. When a singer wishes Larry "eternal life," he becomes upset and disrupts the festival. Later, Mannering arrives at the festival, having followed Larry's murderous path across Europe. The Monster then enters the town and Larry is forced to help it escape. The next day, the citizens once again seek to kill the Monster, but are afraid to wander the unknown catacombs of the Frankenstein castle. Elsa offers to help them search the castle's ruins, and Mannering states that he will kill the Monster using the same artificial means by which it was created. Along with Elsa and Maleva, the physician goes to the castle, and there Elsa unearths her father's secret diaries. From these books, Mannering learns that he can kill the Monster only by reversing its electrical currents. He also concludes that Larry can be killed by artificially draining the energy from his body. The townspeople, however, begin to worry when they see Mannering re-assembling Frankenstein's machinery. The innkeeper, Vazec, proposes that they blow up the dam above the castle and drown all its inhabitants, but the mayor discounts his mad ravings. Despite his promises to Elsa and the townspeople, Mannering refuses to kill the two monsters, and, in fact, reverses the experiment to increase their powers. The full moon comes out, and Larry transforms into a werewolf just in time to save Elsa from the regenerated Monster. The two creatures fight each other as Mannering and Elsa escape the castle, and Vazec blows up the dam. The flood waters destroy the castle, sending the Monster and the Wolf Man, seemingly, to their watery graves.