An alcoholic boxing promoter finds a great fighter, who marries a gangster's girlfriend but with success can't leave the women alone.
A drunk ex-promoter called the Professor (Walter Huston) sees bartender Steve Morgan (Max Baer) bounce a bully and offers him $50 for a prize fight. The Professor trains Steve on the road. A car crashes, and Steve carries Belle (Myrna Loy) and looks after her. He invites her to his fight and knocks out his opponent in the first round. Professor learns that Belle is Willie Ryan's girl, and Steve agrees that she is out. Willie (Otto Kruger) asks Belle about the fight and invites Steve to his table. Belle sings "Down Stream Drifter." Willie tells Steve to leave, and Belle tells him she won't see him again. She says goodnight to Willie outside and finds Steve on the stairs; she lets him come in for a beer.
Willie has the Professor brought in to ask where Steve is. Belle says Steve is in the Professor's office. Willie threatens Belle and Steve. Belle tells Willie she married Steve and returns his jewelry. They shake hands. The Professor coaches Steve, and he and Belle become friends. He gets $1,000 for Steve's next fight because of her publicity. The Professor asks Belle not to honeymoon while Steve is training. She agrees; but Steve eyes a pretty woman. Steve wins again and again until he has won 19 fights in a row. Steve and Belle go to Willie's nightclub. Belle sings, and Steve talks with a blonde and dances with her. Willie talks to Belle about Steve. Belle learns that Steve is lying to her and seeing other women. She cries on the Professor's shoulder. Belle tells Steve she knows but will keep on doing her job. Steve performs with chorus girls on stage. Belle finds Steve with a girl in his dressing room. He says he doesn't care about the girl, and Belle says that's why she won't forgive him.
Belle asks Willie for a job, saying she hates Steve. Steve busts in, and Willie pulls a gun. Belle stays with Willie. Willie by contributing money gets Steve a title fight with Primo Carnera. The Professor tells Steve to stop drinking; but Steve slaps him and says he'll manage himself. The Professor starts drinking again. Famous people are announced at the fight, and Jack Dempsey is the referee. Belle sits with Willie. Steve fights aggressively but is knocked down several times. Belle smiles at Steve and tells the Professor to help him. He yells for Steve to stay away and box him. Steve knocks Carnera down several times. The decision is that the fight is a draw. Willie fires Belle and gives her Steve, who tells her he loves her more than all of it.
When this film was made, Carnera was world champion; he refused to be in the film if he lost. However, the next year Max Baer defeated Carnera to take the crown. The story explores the sexual problems that result from the Professor's insisting on chastity during training and from Steve's cockiness.