During a street battle between gangsters, George Kingsley (Stanley Ridges), a college professor, is injured. He and a notorious gangster, Red Cannon, are taken to the hospital. In an attempt to save Kingsley's life, Dr. Sovac (Boris Karloff) transplants part of Cannon's brain into Kingsley's. The operation is successful, but Cannon dies. To learn the whereabouts of the $500,000 loot Cannon had hidden from his rival, Marnay (Bela Lugosi), Sovac convinces the recovering Kingsley to accompany him to New York. After Sovac hypnotizes him, Kingsley transforms into Red Cannon and goes on a killing spree, targeting Marnay's men. He also visits Sunny Rogers (Ann Nagel), Cannon's girlfriend, and scares her with knowledge of secrets that she had only shared with Cannon.
As Cannon retrieves his hidden loot, Marnay tries to steal it and split it with Sunny, but they fail. Cannon strangles Sunny and coldly suffocates Marnay to death. After the gangster changes back into Kingsley, Sovac returns home with the loot. However, Kingsley suddenly reverts back to Cannon again. Believing that Sovac's daughter had stolen his loot, Cannon attempts to murder her. Horrified, Sovac is forced to kill him. Before being executed in the electric chair, Sovac gives his diary to a reporter, believing that he made a contribution to science.