Adapted from Barre Lyndon's play, a medical doctor studies crime by participating in robberies.
Dr. Clitterhouse (Edward G. Robinson) catches a man stealing jewels from a safe during a party at the Updike's. Police Inspector Louis Lane (Donald Crisp) investigates, but Dr. Clitterhouse leaves to do an operation on Grant (Thurston Hall). Nurse Randolph (Gale Page) sees jewels in his bag, and Dr. Clitterhouse tells her of his thefts and goes to Lane, who tells him about the fences they are watching. Dr. Clitterhouse tells Randolph that he is studying crime by doing it himself. Dr. Clitterhouse finds the fence Jo Keller (Claire Trevor). Rocks Valentine (Humphrey Bogart) tells Jo he left the robbery. Dr. Clitterhouse bets Updike jewels on dice; but police Lt. Johnson comes in, and Dr. Clitterhouse challenges him, winning respect. Dr. Clitterhouse shows the jewelry from four jobs and sells it to Jo for $38,000.
After a job Rocks' gang has their blood tested by Dr. Clitterhouse. Jo comes in to buy the stuff and quarrels with Rocks. Dr. Clitterhouse asks Jo about her conscience and breaks their date. They plan a job, and Jo tells Butch (Maxie Rosenbloom) to watch Rocks. While the gang steals furs, Dr. Clitterhouse examines several men. Rocks locks him in the vault and lowers the temperature. Butch uses a blow-torch to get him out while a tied watch-man hits the alarm. Dr. Clitterhouse arrives late for the split and says good-bye to the gang and Jo, telling her he began to enjoy crime.
Rocks figures out Dr. Clitterhouse's phone number. Dr. Clitterhouse returns to his office and finds Rocks with a gun and his notes. Jo comes in, and Rocks orders Dr. Clitterhouse to work for him; but Dr. Clitterhouse drugs his drink and plans to drown him. Rocks passes out. Inspector Lane learns that Rocks died from the drug. Jo and Butch go to Lane, who questions her. Jo says she killed Rocks and sends Butch to Dr. Clitterhouse; but Lane hears and has Butch detained while he calls on Dr. Clitterhouse and asks about the drug. Lane tells him he has everything but the motive on the murder. Grant comes in while Lane questions nurse Randolph and by phone orders Rocks' gang arrested. Dr. Clitterhouse asks Grant for help and poses his case. Grant says he would get him off as mad. Dr. Clitterhouse admits it is himself, gives Grant his notes, and asks him to represent him. In the trial a doctor testifies, but no one understands him. The jury is divided, and Dr. Clitterhouse answers their questions. The jury finds him not guilty by insanity, and the judge orders Dr. Clitterhouse examined.
Dr. Clitterhouse makes little progress about crime by finding adrenaline in blood but does realize his own growing addiction to the adventures when he lets himself commit murder as a defense against blackmail.