A masked girl sits in a store window in New York, writing cards to demonstrate a fountain pen. Everybody is curious about her because she is so pretty, and she becomes nicknamed "the Angel in the Mask." A certain boy from the country, Bob Singleton, chances to pass the window. He is forlorn because he cannot get work. The masked girl holds up a card, on which is written a word of friendly encouragement. At the boarding-house where the boy is staying a robbery and murder are committed, incriminating evidence is found in Singleton's room, and he is taken to prison. Meanwhile Mary Berthalon is the only person in the house who believes in Singleton's innocence. A few days later the landlady and Dick Strokes pause at the fountain-pen window. She points to a certain pen which she wants, with a finger on which gleams a curiously-wrought ring. Then they enter the store. The masked girl comes out of the window and waits on them. She writes a card to each of them, to the woman, "Thou Shalt not steal," and to the man, "Thou Shalt not kill." Then, snatching off the mask, she reveals herself as Mary Berthalon. Already the police are at the door. Mary has recognized on the landlady's hand a ring which the murdered woman once had shown her, and in Dick Strokes she has intuitively known the real criminal. The guilty pair confess. And Bob Singleton, freed, and landed in a good job, marries Mary Berthalon.