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