The Kid from Brooklyn (1946)

Shy milkman Burleigh Sullivan accidentally knocks out drunken Speed McFarlane, a champion boxer who was flirting with Burleigh's sister. The newspapers get hold of the story and photographers even catch Burleigh knock out Speed again. Speed's crooked manager decides to turn Burleigh into a fighter. Burleigh doesn't realize that all of his opponents have been asked to take a dive. Thinking he really is a great fighter, Burleigh develops a swelled head which puts a crimp in his relationship with pretty nightclub singer Polly Pringle. He could finally get his comeuppance when he challenges Speed for the title.