Adapted from the play by George S. Kaufman and Moss Hart, the son of a banker wants to marry his secretary, who is from an eccentric family.
On Wall Street Anthony P. Kirby (Edward Arnold) tells son Tony Kirby (James Stewart) they have a monopoly on weapons. Martin Vanderhof (Lionel Barrymore) invites clerk Poppins (Donald Meek) to work on his inventions at his house. Vanderhof refuses to sell his house to John Blakely (Clarence Wilson) for $100,000. Penny Sycamore (Spring Byington) is writing a play, and Essie Carmichael (Ann Miller) wants to dance. Paul Sycamore (Samuel S. Hinds) tests fireworks and takes Poppins to the shop.
Mrs. Kirby (Mary Forbes) walks in on Tony with Alice Sycamore (Jean Arthur). Tony tells Alice he wants to marry her. On the street neighbors tell Vanderhof they have to move for a new factory; but he assures them he won't sell. Poppins suggests advertising the revolution in candy. Alice tells her family about Tony. Vanderhof tells Alice of his experience in love. IRS agent Henderson (Charles Lane) calls on Vanderhof, who does not believe in income tax and asks what he gets for it. Tony is welcomed, and fireworks cause Henderson to leave. Essie's dance teacher Kolenkhov (Mischa Auer) comes in. Vanderhof prays before dinner. Tony and Alice go out. Alice says that her grandfather Vanderhof taught them not to be afraid and to do what they want. Tony says he worked on solar power but gave it up to be a banker. In the park kids dance with Tony and Alice. At a restaurant Tony and Alice are laughed at and see his parents. Alice asks Tony to invite his parents to meet her family. Tony screams and starts a panic over a rat.
Penny paints DePinna (Halliwell Hobbes), and Kolenkhov has Essie dance as Tony and the Kirbys arrive a day early. Kolenhkhov suggests that Kirby has ulcers. Vanderhof asks Kirby about unemployment. Donald (Eddie Anderson) rushes in with groceries. Mrs. Kirby says she studies occultism, but Penny calls it silly. Kirby and Kolenkhov wrestle, and the Kirbys plan to leave; but police detectives come in and arrest everyone for revolutionary activity when fireworks explode. They are put in jail, and Kirby complains; but others sing. Vanderhof tells Kirby he can't take money with him. Kirby learns that Blakely arranged the arrest. Vanderhof calls Kirby a failure in life and apologizes, giving him his new harmonica. Attorneys for Kirby arrive, and reporters are alerted. Vanderhof's friends crowd the courtroom. Vanderhof admits making fireworks, and the Judge (Harry Davenport) fines his family $100. Kirby offers to pay it, but Vanderhof's friends collect it. The Judge asks Kirby why he was there, and Vanderhof says he came to buy his house; but Alice says it was to check out her marriage to Tony, and she rejects the Kirbys as snobs.
Tony searches for Alice, but her family won't say where she is. Vanderhof brings in a letter from Alice. Vanderhof calls Blakely to sell the house for $25,000. Neighbors get vacating notices. Kirby orders that Tony must be president. Defeated Ramsey (H. B. Warner) tells Kirby he is broke and warns him. Tony tells his father he is quitting and leaving. Kirby learns that Ramsey died of heart failure. Kirby turns away from his board. Vanderhof's family packs to move, and Tony tells Vanderhof that he loves Alice, who comes in and avoids Tony. Kirby comes in and asks Vanderhof for advice. Vanderhof has Kirby play harmonica with him. Tony and Alice see them and embrace and dance. In the final scene Vanderhof prays before dinner at their house with the Kirbys.
This Oscar-winning picture challenges excessive capitalism by contrasting it with happy Bohemians pursuing their own interests.