When pretty, hungry Parisian Arlette Lefon inquires about work at Professor Aristide's "professional school," she is unaware that it only provides training for pickpockets. Although Aristide at first rejects Arlette for being too honest, he changes his mind when he finds out that she is a reform school runaway. Under Aristide's exacting tutelage, Arlette quickly learns the various skills necessary to be an effective pickpocket. Despite her educational successes, however, Arlette confides to fellow pupil Yves Cadubert that she intensely dislikes the idea of stealing. The sympathetic Yves tells Arlette that marriage is the only honest way to avoid reform school and offers to connect her with a friend who arranges marriages for a 3,000 franc fee. Struck by Yves suggestion, Arlette declares that she will steal the 3,000 francs to stay "honest" and pickpockets a pearl tie pin from a distinguished man on the street. The man, however, is aware of the theft and corners Arlette in a movie theater. Instead of reporting her to the police, the man, an ambassador, takes her to his lavish home and forces her to dress in a ball gown. The ambassador then blackmails his adulterous colleague, Baron Dvorak, to pose as Arlette's uncle and escort her to a diplomatic ball. There the ambassador orders Arlette to dance with handsome junior diplomat Pierre and steal his pocket watch. Before she gives the stolen watch to the ambassador, Arlette notices it contains a photograph of the ambassador's wife. When the jealous ambassador inspects the watch, however, he is relieved to find it empty and tells Arlette to return it to Pierre. Impressed by Arlette's youth and beauty, the unsuspecting Pierre insists on driving her to her "uncle's" home and kisses her on the moonlit street. Although intoxicated by Pierre's charms, Arlette tells him that she is leaving for Geneva the next day and is stunned when he gleefully announces that he is going there on the same train. Back at the professor's, Arlette lies about her activities and is accused of a double-cross. After Aristide throws her out, Arlette rushes to the train station and tells Pierre the truth about herself. Unaware that Arlette removed the photograph, Pierre runs to telephone the ambassador's wife. While he is talking, he misses his train, but the lovestruck Arlette recovers his dog and his baggage and returns them to his house. Feeling sorry for the runaway, Pierre invites her to stay with him and then gets the idea to pay his perpetually broke friend, Roland Medeville, 10,000 francs to marry her "in name only." Although Pierre arranges for the phony marriage to take place while he is in Geneva, he later regrets his actions and returns prematurely to Paris. To his astonishment, he discovers that Arlette has been transformed by Roland into a sophisticated woman. When a gushing Roland tells Pierre that he now wants more than a marriage in name only, Pierre confesses that he loves Arlette. Roland convinces him to drop the romance, however, by reminding him that it would ruin his diplomatic career. Once at the altar, however, Arlette says no to Roland and prepares to return to the street. Yves, who has been working as Pierre's butler, assures Arlette that Pierre loves her, and she determines to prove herself worthy of his station. Dressed in the gown that the ambassador gave her, Arlette resumes her impersonation of the baron's niece at another diplomatic ball. There Pierre is further impressed by Arlette's transformation, and while the chagrined ambassador forces the baron to agree to adopt Arlette, Pierre proposes. Their respective dilemmas thus resolved, Arlette and Pierre marry.