After Sir Karell Borotin is found dead in his study, drained of his blood, the local doctor determines that he was killed by a vampire. The coroner and police Inspector Neumann dismiss the suggestion but a year later, Sir Karell's daughter is attacked and bite marks are found on her throat. Neumann calls in the eminent Professor Zelin who thinks the story of vampires is true. The locals are convinced that Count Mora and his daughter Luna are the perpetrators of the crime, creatures of the night that can turn themselves into bats. There may be another solution however and the Professor sets a trap.