Two of Ma O'Hara's three sons have solid jobs. Mike is a fireman and Pat is a policeman, but Danny, Ma's favorite because he is the youngest, is an occasional fight promoter who has never handled a successful boxer. Pat tries to talk Danny into taking the police exam, because he is planning to marry Lucille Jackson, his captain's daughter, and is worried that the family will not be able to get along without his salary. Danny is not interested, however, having discovered a new fighter, Carbarn Hammerschlog, who comes out slugging whenever he hears a bell. Anxious for Lucille to meet his mother, Pat asks her home to dinner, but Danny meets her first, by accident, when he and Carbarn help her change a flat tire. Lucille and Ma become friends right away, but Lucille causes her some distress when she admits that Pat has not asked her to marry him. When Pat tries to change out of his uniform, he discovers that Danny has loaned his best suit to Carbarn and then, to add insult to injury, he happens to be standing next to Carbarn when a bell rings and Carbarn knocks him out. With the dinner ruined, Danny offers to take Lucille home, and they stop along the way for an intimate dinner. Later, Lucille agrees to be Pat's date at the Fireman's Ball, but she cannot stay away from Danny. Pat sees them kissing and is so hurt that he plans to leave home until Danny moves out himself. Upset at having come between the two brothers, Lucille visits Danny at the gym to tell him she is not in love with him. Danny finally gets Carbarn a fight with the champion. Before the fight, Carbarn gets drunk while trying to kill a toothache with a bottle of gin, and Danny must go on in his place. He takes a beating, but keeps on fighting. At Ma's insistence, Pat gets into Danny's corner to tell him that Lucille does love him. Inspired, Danny defeats the champ and is reunited with his brother and Lucille.