Footsteps in the Sand (1939)

A girl is murdered and the evidence points strongly toward Peter Charrington, who disappeared right after the crime. His wife Mary isn't thoroughly convinced that Peter isn't guilty but she loyally hides him after he sneaks back to their home. Developing circumstances show that other murders have been committed under the same circumstances, of a full moon. Mary begins to gather pieces of evidence that the murderer is her attorney, Lawrence Crawford, seized by moments of moon-madness.