Two gangs of jewel thieves compete for pearls while an under-cover cop falls in love with the woman he hopes will lead him to them.
Vivian Palmer (Myrna Loy) has a car accident in order to meet Wadsworth (Robert Warwick). Wadsworth buys four pearls for $500,000, but they are stolen from him by Palmer's partner Ed Dexter (Harvey Stephens). In a bar Ross McBride (Spencer Tracy) gives Palmer money and is searched by a cop, who mentions his record. She invites Ross to her apartment and accepts none of the money. When the cop comes in, Ross hits him, handcuffs him, and leaves him in the closet as Palmer packs to go, telling Ross that thieves are after her. They take a plane and get off in St. Louis, trying to lose two men following. Ross invents a honeymoon story and sleeps on the couch in their hotel room. Palmer goes out to call Dexter that she knows Ross is police. Dexter planted the pearls in the handle of her mirror, but she doesn't know it. Ross tells her he will get rid of the men following but tells police not to pick them up so that she will lead him to Dexter.
Doc Evans (William Harrigan) learns that Dexter is in New Orleans. Ross tells Palmer he wants to use Dexter for a jewelry heist. At night she sneaks out but finds Ross on the street. They drive in the rain to a house, and the man asks Ross to go for a doctor, who delivers his wife's twins. Doc Evans and his gang break in on Dexter with guns. Ross and Palmer care for the twins, transforming their attitudes. Palmer stays on the couch, but Ross carries her to the bed before laying on the couch. Palmer calls Dexter in order to quit. Ross calls in Dexter's phone number, letting Palmer know he is a federal agent and arresting her. Ross feels whipsawed and wants to help her, and they kiss; but she drops her mirror, releasing the pearls. Ross turns hard again and cuffs her wrist to his.
Ross throws the pearls out the window before Doc Evans' gang and Dexter come in. Palmer says that Ross is not a cop but her husband, and Dexter tells her he planted the pearls in her mirror. Ross says he gave the pearls to a pal. They all go in the car and stop for food. Ross puts dirt on Palmer's face so she will leave the room. Then he pulls a gun to arrest them, shooting two before he is shot. Police come in and arrest them. Palmer is in jail and is taken to the recovering Ross, who questions her and says he is her witness. Ross turns in his badge. Palmer questions him and says yes.
The two principals make this a charming story that shows love transforming a jewel thief and a tough detective into giving up their professions to please each other.