Sir Borotyn, a prominent Prague resident, is discovered murdered in his home, with all indications pointing to a vampire assault. The victim's friend, Baron Otto, and the physician who analyzes the body are certain that the vampire is the mysterious Count Mora, or perhaps his daughter, but receive little help from the law. Professor Zelen, an expert in the occult, is called in to assist with the investigation.

Mary Barrett is an aspiring opera singer who is taken under the wings of a famous operatic maestro, Guilio Monterverdi. After spending endless working hours together and arguing, their relationship develops into love. But, jealousy and misunderstandings prevent Mary and Guilio from acknowledging their true feelings.