Mabel Harris, a society girl, is known as "the angel of the slums." She is loved by Charles Brandt, the doctor of the poor, but already engaged to a wealthy man, Frank Melville, whose office boy, Billy Tanner, is the sole guardian of his sick baby brother. The slum doctor gets Mabel interested in Billy and the baby, but she does not know that Billy is Melville's office boy. She tries to get Frank interested in her work and their quarrels start with his belittling her efforts. Billy returns home from work one day to find Mabel being insulted by a tough young gangster. Billy grabs a chair and would have hit the gangster with it, but for the timely interference of a neighbor. The tough threatens vengeance and leaves with the neighbors while Mabel thanks Billy. She meets the doctor and tells him she thinks Billy will be injured by the man. They both return to find the door locked. The gangster has returned and is in the house with Billy behind a locked door.
