The Hardy family travels to Washington for the Judge to head a commission, but complications arise from romances of his son and daughter.
Judge James Hardy (Lewis Stone) sentences three students protesting for a football player to writing an essay. J. J. Harper tells the Judge of his appointment as head commissioner on a public utilities hearing in Washington. Andy Hardy (Mickey Rooney) asks Polly Benedict (Ann Rutherford) to walk home with him, but she asks him to wear a tuxedo to the dance. Wayne Trenton (Robert Whitney) tells Marian Hardy (Cecilia Parker) that he wants to marry her sooner. Mrs. Emily Hardy (Fay Holden) tells James $200 a day is not honest but agrees to go.
In Washington a car picks up the family, and they dine in the Embassy room, where Andy meets French Suzanne Cortot (Jacqueline Laurent), and John Lee (Jonathan Hale) and Margaret Lee (Ruth Hussey) meet the Hardys. Andy finds Suzanne walking with her tutor, Miss Budge (Janet Beecher). The Lees take Marian to the Senate and lunch. Marian discusses utilities policy with her father. After Marian goes to a costume ball with Steve Prentiss, her parents send for Wayne. Andy goes to a museum with Suzanne, and she invites him to a formal party.
Wayne arrives, but Marian quarrels with him. James tells her he sent for Wayne as an engineer. Harper tells James that Courtney Power Co. stock went up and warns him about leaks. Andy asks his father for $20 for a tux. After Andy buys it, his father finds out he wrote the protest leaflet. He takes Andy to Mt. Vernon and the Declaration of Independence, forbidding him to wear the tux for thirty days. At dinner the Judge lets Andy go to the party. Suzanne asks Andy to dance, and he gets the orchestra to swing so they can dance fast until the matron stops it. Miss Budge tells Andy he can't see Suzanne again. John Lee calls on Judge Hardy for Courtney Power and threatens to expose Marian's leaks from the hearings; the Judge calls it blackmail. Marian comes in crying because of a gossip column about her. Her father tells her that the shock of failure can get one on the right track.
On the train Andy leaves Wayne and Marian alone to kiss. Aunt Millie (Betty Ross Clarke) asks James about the possible investigation. James writes a letter resigning as judge. Emily and he reminisce, and she is ready to start again. Andy gets a top hat sent by Suzanne. Andy tells his father he plays hard to get and accuses them first. James gets an idea, and on radio he says how he used his daughter to expose the illegal monopoly of Courtney Power. Polly speaks French and likes culture, and Andy kisses her.
While preserving his integrity in standing up for democracy against corruption, the Judge teaches his children his values while letting them learn from experience.