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. After Mike leaves and gets a job making artificial flowers, she writes DeWitt daily letters, signing them "Bianca," which inspire him to write a best selling novel of replies. Although DeWitt nearly marries Judith when she claims authorship of the letters, he weds Mike when he learns that she penned them.