NOTE: Sequel to "The Devil Bat" (1940).
An unconscious woman is found lying in the road near the old Carruthers house. She turns out to be Nina MacCarron [Rosemary La Planche], daughter of Josephine MacCarron and Dr Paul Carruthers, known as "the devil bat" for his work in cell growth stimulation, in which he created gigantic killer bats to murder those he felt had wronged him. The town doctor, Dr Elliott [Nolan Leary], is called in to treat Nina, but he asks for help from New York psychiatrist, Dr Clifton Morris [Michael Hale]. Dr Morris brings Nina out of her unconscious state and recommends that she be placed in a hospital for a few days of observation and R&R. That night Nina has a dream about a bat fluttering outside her window. She panics and runs from the hospital--straight to the house where Morris lives with his wife Ellen [Molly Lamont]. Unfortunately, Morris is in New York with his paramour, Myra Arnold [Monica Mars]. Myra wants Morris to divorce Ellen and marry her, but Morris refuses, although he admits to marrying Ellen only for her money and position. The next day, Morris returns home to find that Ellen has put up Nina in their guest room. He continues to work with Nina there, delving into her memories of her parents. Whenever she is questioned about her father, however, Nina gets agitated and blocks the memories. Still, it's very apparent that she associates her father with bats.
Over the next month, they make some progress. Nina begins to remember her Romanian father (played in "Devil Bat" by Bela Lugosi) who left her mother when Nina was 4 years old. She also recalls some of the stories that people used to tell about him being a vampire. She recalls her dreams, in which he appears to her in the form of a bat. She remembers going to his lab and finding an old newspaper saying that he was a murderer. Morris assures her that her problems stem from stresses during the war, finding that her father is dead, and feeling that she is alone in life. When Nina recalls feeling her father's presence while visiting the lab, Morris assures her that this was just her imagination, because "the dead do not return." Dr Elliott explains that a vampire is a dead person who returns to the living and must drink blood in order to maintain his power. He also describes a medical condition in which a living person believes that s/he is possessed by a vampire and must do its bidding, even if it means killing. This appears to be Nina's problem.
Ellen's son Ted Masters [John James] suddenly returns from the army. A budding lawyer, Ted falls in love with Nina, even though she is a patient of Dr Morris, Ted's stepfather. It's just a case of nervous exhaustion, according to Morris. But Nina is still having dreams of bats and of her father, so Morris increases her medications. One morning, the family dog is found dead in Nina's bedroom, its throat cut by a scissors. Morris is convinced that Nina's vampiric possession has gotten the best of her. Even more damning is the fact that Nina's father was said to be a "vampire murderer" and it's likely that Nina has inherited his "criminal tendencies." Morris makes plans to place Nina in a private sanitarium. The next morning, Ellen is found murdered in her bed and Nina is found lying at the bottom of the stairs outside Ellen's bedroom door with a bloody scissors in her hand.
It's six days until Nina's trial. Ted decides that he has to try to clear Nina, so he and Dr Elliott go to the old Carruthers house in hopes of finding Carruthers' research, which has turned up missing. They find the house has been ransacked and the safe is empty, but it's when Ted finds Dr Morris's Chinese "worry ball" on the lab floor that he puts together 2 + 2. Ted searches the Morris house and then Morris's New York apartment. All he finds is a pill that Morris had prescribed for Nina and a letter that leads him to Myra Arnold. A visit to Myra reveals that she and Morris have been carrying on an affair for quite some time. Also worried that Morris might have killed Ellen, Myra offers to let Ted search her apartment in case the papers are there, which they are. Ted confronts Morris with the worry ball, the research papers, and the suspicion that he killed Ellen and then tried to blame it on Nina. When Dr Elliott and the sheriff arrive, Ted explains why Morris didn't hand over the papers -- because they proved Carruthers was NOT a murderer. He was a great scientist, and it's unfortunate his bats got loose and killed a few people, including Carruthers himself. If Nina had found out the truth, Ted explains, she would have been cured, which was not part of Morris's plan.
But the final proof comes in the form of the pill that Ted found and had analyzed at a medical laboratory. It's a "sodium derivative," a powerful dream stimulant often used in psychiatry. Ted accuses Morris of using these pills along with the power of suggestion to drug Nina night after night into believing that she was the killer. When Ted also reveals that this medication renders the taker incapable of movement, it becomes apparent to everyone that Morris must have carried Nina downstairs and planted her outside Ellen's door after *he* killed Ellen. When Dr Morris sees that he has no way out, he pulls out a gun, fires a few shots, and runs out to the terrace. The sheriff shoots him. As he dies, Morris whispers to Ted, "Forgive."
Now that the truth is out, Nina is cured. [Original synopsis by bj_kuehl.]