Based on Compton MacKenzie's novel, two smugglers and a daughter dressed as a boy wander England conning and then doing shows until the jealous father commits suicide and his daughter falls in love with an artist.
In Marseilles Henry Scarlett (Edmund Gwenn) is mourning his late wife and being investigated for embezzlement. To help escape, his daughter Sylvia (Katharine Hepburn) dresses as Sylvester. On the ship to England Henry shows Jimmy Monkley (Cary Grant) the lace he is smuggling, and he is fined by customs. On a train Sylvester hits Jimmy, but Jimmy gives Henry 100 pounds and shows the gems he smuggled. The three agree to travel together. In London Sylvester's crying draws a crowd. Henry translates his hard-luck story, and Jimmy gives the boy a pound. Soon many are giving until Sylvester blurts out in English that someone took some of the money.
Jimmy says they would do better with a girl. They go to an estate of a vacationing family. Jimmy cajoles the maid Maudie Tilt (Dennie Moore), and Henry gives her a singing audition. Sylvester climbs in the window. Jimmy lifts jewels off Maudie, but Sylvester makes him give them back. Maudie offers her savings to back their shows and kisses Sylvester. Michael Fane (Brian Aherne) laughs at the show and invites them to his studio. He blames Lily Levetsky (Natalie Paley) for making the four leave. Sylvester below Michael's window apologizes to him and pays for things broken. At a cave by the beach Sylvia changes into a dress and calls on Michael. He sees she is a girl and laughs. She cries about her hair and freckles; but he gives her a new entrance. Lily arrives apologizing. She tells Sylvia that Michael loves her; but Michael tells Lily he adores her, kisses her, and tells her to stay. Sylvia says good-bye and runs into Jimmy. He says he likes her. Jimmy tells Maudie to pack, and Henry becomes jealous of Maudie. In the rain Henry goes looking for her. Sylvia and Jimmy look for Henry, and they find his body below a cliff.
Jimmy suggests to Sylvia they go on together, and he tries to kiss her. She hears Lily calling for Michael from the ocean. Sylvia swims out and saves her. Sylvia tells Jimmy that Lily must love Michael. Sylvia tells Michael what happened, and they find the caravan gone. Sylvia and Michael follow. Two women take their dress back from Sylvia. A constable asks for her license; Michael and Sylvia in men's clothing are put in jail. On a train Sylvia and Michael find Jimmy and Lily; but they realize they love each other, stop the train, and run off into the woods. Lily quarrels with Jimmy, saying she will go back to Michael; but Jimmy just laughs.
Naturally attracted to women, Jimmy admits to Sylvia it is a "pig's world." Henry could not handle his jealousy; but after dubious ventures with her father, Sylvia finds Michael.