Dr. Paul Carruthers is frustrated because he thinks his employers, Mary Heath and Henry Morton, have cheated him out of the company's profits. He decides to get revenge by altering bats to grow twice their normal size and training them to attack when they smell a perfume of his own making. He mixes the perfume into a lotion, which he offers as a gift to Mary and Henry. When they turn up dead, a newspaper reporter decides to investigate.

Wishing to dispose of his wife, psychiatrist Dr. Elliott makes his patient Nina think that she suffers from a compulsion to kill. He drugs Nina, murders his wife and leaves evidence that points to Nina. The latter, pre-conditioned by Elliott, also thinks she is guilty.