Its plot follows a series of deaths and attacks by vampires that brings eminent expert Professor Zelen to the aid of Irena Borotyn, who is about to be married.

Her father, Sir Karell, died from complete loss of blood, with bite wounds on his neck, and it appears he may be one of the undead now plaguing the area.

The attending doctor, Dr. Doskil, and Sir Karell's friend Baron Otto von Zinden are convinced that he was killed by a vampire.

After Irena is menaced by the vampires on several occasions, Zelen, Baron Otto, and Inspector Neumann descend into the ruined parts of the castle to hunt down the undead monsters and destroy them.

It is then revealed that the "vampires" are actually hired actors, and that the entire experience has been an elaborate charade concocted by Zelen in the hopes of tricking the real murderer —Baron Otto— into confessing to the crime.

Acknowledging that the charade has failed to produce its intended results, Zelen, along with Irena and another actor who strongly resembles Sir Karell, compels the hypnotized Baron into re-enacting the murder, effectively proving his guilt.

With Baron Otto arrested, Irena explains the plot to her fiance, Fedor, who was not involved in the subterfuge and believed that the vampires were real.

[6][7] Makeup artist William J. Tuttle would later recollect to author Richard Bojarski, The crew and I didn't like to work for director Tod Browning.

I remember he gave the special effects men a hard time because they weren't working the mechanical bats properly.

She reports that an alternate ending – in which Professor Zelen receives a telegram from the hired actors revealing that they were unable to make their train (thus implying the vampires that the film depicts were real) – was considered but rejected by Browning.

[1][2] Several sources, including critic Mark Viera[9] and Turner Classic Movies writer Jeff Stafford,[6] have claimed that MGM cut out suggestions of incest between Count Mora (played by Lugosi) and his daughter Luna.

[11] Lugosi biographer Arthur Lennig asserts that Endore did originally intend an incest backstory, but that it was removed by the studio before the shooting script was written, though he goes on to claim that a cut line of dialogue indicates that Count Mora shot himself after strangling his daughter.

[12] In the commentary which accompanies the film on the Hollywood Legends of Horror boxed set released by Warner Home Video, genre historians Kim Newman and Steve Jones seem to concur with Mank, hypothesizing that primarily comedic material—possibly related to the maid character played by Leila Bennett—was cut.

[16]Seymour Roman of the Brooklyn Times-Union praised the film as "a gloriously unrestrained example of the cinema [of the] supernatural, thrilling, chilling, and horrifying.