Mike Kildare, a swaggering youth from New York City's Bowery at the turn of the century, comes to the defense of Mamie Rose, a mender in a secondhand clothing shop, when his own gang of Irish-Americans insult her. Mamie is fascinated by Kildare's brute strength, but she is also attracted to Max Davidson, the kind and gentle nephew of her Jewish tailor boss, Old Levi. Kildare's gang, which Mike has forsaken to prove his love to Mamie, gives him a severe beating in an underground den, but he takes the punishment, reforms, wins Mamie's hand in marriage, and joins the police force.