In New York City's Bowery in the early 1900s, former world heavyweight champ Roy "Slag" McGurk wonders why Johnny Burden, the boy he "brought out of the gutter" and trained to be middleweight champion of the world, has turned his back on boxing and joined the Salvation Army. While Slag works at Mike Glenson's saloon bouncing some of the neighborhood's roughest customers, Johnny devotes his life to "saving souls" in the Bowery. Johnny also devotes himself to continuing his romance with Glenson's daughter Caroline, even though her hard-nosed father disapproves of him and the Salvation Army. Glenson rents out part of his building to the Salvation Army, which refuses to move, and sees the organization and Johnny as obstacles to his expansion plans. To prevent Johnny from resuming his romance with Caroline, who is returning to New York from London, Glenson sends Slag to Ellis Island to be the first to greet her. Slag remains loyal to Johnny, however, and tips him off about Caroline's arrival. At Ellis Island, Slag inadvertently becomes the temporary guardian of a young English orphan named Nipper, who has been sent to America to live on Madison Avenue with his wealthy uncle. As the boy has lost his uncle's address, Slag consents to help him find his new home, hoping to collect a generous reward for his efforts. Back in the Bowery, Glenson threatens to expose Slag's false claims as a fighter unless he agrees to incite a riot in front of the Salvation Army. Nipper, meanwhile, has grown fond of Slag and asks to stay with him permanently, but Slag eventually discovers that the boy's uncle is Bruno Milbane, Vice President of Milbane Investment Co., and resolves to deliver the boy to his rightful guardian. Milbane and his associates, Fowles and Flexter, are actually underworld figures who are skipping town and want nothing to do with the boy, so they promise Slag a cash allowance to keep the boy for a time. Slag accepts the offer because he is desperate for money to buy back his fight belt from his old sweetheart, Mamie Steeple, who runs a pawnshop in the Bowery. Later, Johnny tells Slag that he will withhold his recommendation of him to be the boy's guardian to the Childrens Protective Society until he vows to stop drinking and brawling, and until he joins the Salvation Army. Still hoping to profit from keeping Nipper for Milbane, Slag immediately gives up his vices and joins the Salvation Army. When Slag learns that Milbane and his associates are broke and do not want Nipper, he loses interest in the boy and takes him to the Salvation Army. There Nipper tells Mamie that Slag only kept him because he was promised a reward, which angers Mamie and prompts her to upbraid Slag. Slag then has a change of heart and, after confessing to everyone in the saloon that he won the heavyweight championship only after his opponent took a fall, he leads the fight against those attempting to start a riot at the Salvation Army. Slag wins back the respect of those he loves and Johnny recommends him for Nipper's adoption. While Caroline and Johnny resume their romance undisturbed, Slag and Mamie look forward to rearing Nipper together.
