Dr. Clitterhouse, a successful and respected Park Ave. doctor, decides the best way to study the criminal mind is to become a criminal. He begins by robbing the safes of his wealthy friends, but soon wants to observe the underworld more closely and test his theory that criminal activity changes the personality. He seeks out Jo Keller, a well-known fence, and is surprised to discover that Keller is a beautiful woman. While the doctor's devoted nurse, Randolph, covers for him, Clitterhouse joins the gang of criminals run by Keller and Rocks Valentine. Jo falls in love with Clitterhouse, now nicknamed "the Professor," but Rocks is distrustful of the doctor's motives and, one night during a fur heist, tries to kill him by trapping him in a cold storage locker. Once the doctor has concluded his experiments on the gang, he goes back to his practice. Rocks trails him, discovers his notebooks, and threatens blackmail. Clitterhouse, who has now completely lost perspective on his own behavior, murders Rocks and with Jo's help hides the body. When Inspector Lane closes in on Clitterhouse, the doctor confesses his crime to a prominent attorney named Grant. Clitterhouse is tried for murder, but is ultimately aquitted on the grounds of insanity, not because of his criminal actions, but because when he is put on the witness stand he undermines his own defense.