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.