Dr. T.S. Clitterhouse (Edward G. Robinson), a wealthy society physician, decides to research criminal behavior by becoming one himself. He joins a gang of thieves led by "Rocks" Valentine (Humphrey Bogart) and proceeds to wrest leadership away from the resentful leader. As part of his research, Dr. Clitterhouse pretends to be an expert thief, impressing Jo Keller (Claire Trevor), the biggest fence in the city, with his exploits, leading her to invite him into the gang.