Tragedy strikes when Dr. Ernest Sovac’s (Boris Karloff) best friend, college professor George Kingsley (Stanley Ridges), is run down while crossing a street.
In order to save his friend’s life, Sovac implants part of another man’s brain into the professor’s. Unfortunately, the other man was a gangster who was involved in the accident. The professor recovers but at times behaves like the gangster, and his whole personality changes.
Sovac is horrified but also intrigued, because the gangster has hidden $500,000 somewhere in the city. The doctor continues to treat his friend and, when the professor is under the influence of the gangster’s brain, Sovac attempts to have the man lead him to the fortune.
Béla Lugosi appears as gangster Eric Marnay – another man attempting to get his hands on the cash.