Happy O'Brien, called "The Microbe," or "Mike" for short, a female street urchin who sells newspapers dressed as a boy to avoid harassment, is saved from arrest for fighting other newsboys in Chinatown by wealthy author DeWitt Spense. DeWitt, there to purchase drugs, is so moved by Mike's pugnacity, that he takes her to his mansion for literary inspiration. When DeWitt learns that Mike is a girl, he resolves to educate her. As Mike blossoms, DeWitt's attentions turn to love, to the dismay of Judith Winthrope, who wants to marry him, and his friend Robert Breton, who, with Judith, convinces Mike that DeWitt's writing is suffering because of her.

Viola Dana
as Happy O'Brien, The Microbe

Kenneth Harlan
as DeWitt Spense

Bonnie Hill
as Judith Winthrope

Arthur Maude
as Robert Breton

Ned Norworth
as Norman Slade

Lucy Donahue
as Mrs. Risden