Dr Paul Carruthers [Bela Lugosi] appears to all of Heathville as a kindly doctor and research scientist for Heath-Morton Cosmetics, Ltd. But in his secret laboratory, Carruthers is actually conducting personal experiments--one of which is a formula for an oriental shaving lotion; another is the creation of a giant bat who hates the smell of the lotion. Bitter because founding fathers Martin Heath [Edward Mortimer] and Henry Morton [Guy Usher] got rich on his formulas while he himself is awarded with menial bonuses, Carruthers plans to sic his giant bat on their families.
First to go is Roy Heath [John Ellis]. Investigative reporter Johnny Layton [Dave O'Brien] and cameraman 'One Shot' McGuire [Donald Kerr] of the Chicago Register are sent to snoop for a story. The only clues are the victim's neck wounds that look like they were made by the beak and talons of a bird, a peculiar faint odor about the wounds, and some mouse or bat hairs. That afternoon, Layton and McGuire have tea with Carruthers and Martin's daughter Mary [Suzanne Kaaren], and Layton and Mary become smitten with each other. Mary invites Layton to come back to the estate that evening.
That evening, Johnny sees Martin's son Tommy [Alan Baldwin] attacked in the garden by a giant bat. When Don Morton [Gene O'Donnell], son of the other founding father, Henry Morton, is subsequently killed in the same way, Layton finds a bottle of shaving lotion of the same scent discovered on all the bodies. In an attempt to provide a picture of the 'devil bat', McGuire photographs a fake bat which is proven fraudulent by a scientist on a talk radio program. Layton and McGuire are fired by their paper, but they stay in Heathville to get to the bottom of the story. They trace the lotion to Carruthers and interview him in an attempt to expose him, but he readily admits to creating the formula. He even gives Layton a bottle to try. That night, Layton and McGuire are dive-bombed by the giant bat, which Layton kills. The size of the bat has scientists marvelled, and Layton and McGuire are exonerated and rehired.
Carruthers creates a new devil bat and sics it on Henry Morton. That night a giant bat attempts to get into Mary's bedroom, and it is learned that someone has switched her perfume with formula. While Carruthers tends Mary, Layton visits his laboratory. He waits for Carruthers to return and follows him into his secret lab where Carruthers releases the devil bat. Layton asks Carruthers for a bottle of the lotion and informs him that he intends to sit in the garden and hopefully attract the bat with it. Layton even invites Carruthers to watch, but he splashes lotion on Carruthers and the bat ultimately kills Carruthers. [Original synopsis by bj_kuehl.]