Near the end of the 19th century, Prof. John Mayer and his assistant, the scientist Dr. Isabel Reed, accidentally invent a powerful ray. An alien travels to Earth to destroy the machine since it would be a threat to the universe. He initially possesses the mind of Thomas, a sex maniac and serial killer, to get close to Mayer during the demonstration of the machine to scientists and the military. Mayer's niece, Laura, meets an acquaintance of her uncle, the chemist Dr. Paul Rosten, and Mayer invites him to join the research. When Thomas gets close to Mayer, the alien also possesses his mind in order to destroy the device and his notes. Meanwhile, Thomas is not able to control his murderous impulses and attacks several women with a straight razor.

Marty Mackenzie is an unsuccessful stage actor who takes an interest in private investigating. He takes a job working with Jack Potter, a crusty private eye. They both take a case in Beaver Ridge, a seedy small town where a murder is being planned against a rich gravel pit owner. Marty realizes that private investigating is not as it seemed to be.