Dr. Ernest Sovac is taken from his cell for his execution, but is able to give notes to a reporter, which recount his story, as he is led to a chamber.

Sometime earlier, Sovac's best friend, bookish college professor George Kingsley, 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 and was apparently heading for the electric chair, according to the police. The professor recovers but at times behaves like the gangster. Sovac is horrified but also intrigued, because the gangster has hidden $500,000 somewhere in New York City. The doctor continues to treat his unwitting friend and persuades him to take a vacation in New York; Sovac hopes this will revive the gangster's memory so that Kingsley will lead him to the fortune which he hopes to spend on a laboratory. Unfortunately, for the doctor's plans, the professor's personality change becomes more extreme, including plotting revenge against other members of his former gang. When Kingsley (behaving as a gangster) attempts to murder the doctor's daughter, Sovac shoots him dead.

Returning to present, Sovac is executed.