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. Billy evades him until he threatens the baby's life and then gives in to him. As he does, the doctor smashes in the door and pounces on the gangster. A fight follows in which the doctor scores a couple of knock downs and finally kicks the bully out of the room. Mabel's warm praise starts the doctor further on the path of love and his face shows so plainly his love for Mabel that she, embarrassed and rather pleased, turns to Billy with an admonition to be careful, then exits with the doctor. Frank has been waiting for her at her home and gets peeved at her long absence. Mabel and the doctor part with regret and the quarrel between the engaged pair is furthered by Frank taking Mabel to task again because of her slum work. The next day Billy stays with his brother longer than he should because the child is too ill to be left even with a neighbor. Mabel enters and when the doctor comes he finds her very maternally attending to the baby's wants with Billy looking on. When they are ready to leave, Mabel exits with the doctor. This time their parting is more difficult and shows increasing love. When Mabel arrives home she finds a note from Frank with a luncheon request and Mabel decides to go. Meanwhile Billy has left for the office. Mabel finds Frank in a grouch at the boy because he is late and she warns him that if he is going to be grouchy that will be the end of the luncheon. Billy arrives and is called upon the carpet. He does not see Mabel standing in the window. Frank scolds the boy. He is discharged and Mabel enters the argument by giving Frank back his ring and breaking the luncheon engagement. She takes Billy home and there finds the doctor. Billy brings about an avowal of love on the doctor's part, which Mabel accepts. Billy is told he and the baby will have a home for life, and the angel of the slums finds her happiness.