Scrappy choirboy Tommy McCoy earns pennies singing at the pool hall, hoping to help his hard working mother. His father Brian, a shirking former vaudevillian, decides to have Tommy sing before a boxing match, and when a member of the crowd goads him, also has Tommy box the man's son. Because Tommy is successful at fighting, too, champion Johnny Martin sponsors him on a road trip. Brian gambles and drinks away their money, even though Tommy tries to send some home. When Tommy's mother Laura dies, Brian regrets his behavior, but doesn't change. Many years later, Tommy is a successful boxer. When his old friend Johnny comes out of retirement, Tommy tries to go easy on him in a match, but Johnny is accidentally killed from one of the blows. Although Martin's wife knows that Tommy is blameless, the newspapers call him "Killer" McCoy. His guilt over Johnny and his estrangement from Brian after Brian sells his contract to gambler Jim Cain makes Tommy decide to quit the ring. He tries to find other work, but can't, so he eventually goes back to Cain, who promotes him as a fighter who wins on "lucky" punches, and wins alot of money on long odds. In training at an estate owned by Cain under his real name of "Carson," Tommy meets his daughter Sheila. They fall in love, but, as Sheila knows nothing about her father's business, Cain is against their relationship. They continue to meet secretly and Tommy plans to give up fighting, but, just as he is about to enter the ring in a championship fight, he receives word that Sheila and Brian have been kidnapped by rival gambler "Pug" Walsh. Walsh expects Tommy to lose in the eighth round, but wants him not to look like he is throwing the fight. Tommy holds out, despite a terrible beeting, but finally knocks out his opponent when he sees that Sheila is all right. Brian, who had feigned a heart attack to help her escape, is killed by one of the kidnappers. Realizing that they love Sheila more than they dislike each other, Tommy and Cain become friends and Sheila promises to help reform them both. Finally, Tommy and Sheila are married.