While conducting research on the physical impact of crime, Dr. Clitterhouse (Edward G. Robinson) decides to become a criminal himself. After a few jewelry thefts, the doctor meets Jo Keller (Claire Trevor), a major figure in the criminal underworld. He takes a liking to Keller and becomes a member of her gang, supplanting "Rocks" Valentine (Humphrey Bogart). Once he has enough data, Clitterhouse tries to return to his normal life, but Valentine finds out his identity and blackmails him.