A small-town tuba-playing bumpkin named Longfellow Deeds (Gary Cooper) learns he has inherited a fortune from an uncle he never knew. He must go off to Manhattan to claim the money and runs into a nest of vipers. Most take advantage of him and want to strip him of control of the estate. Local media hire a sneaky woman (Jean Arthur) to write stories to make Deeds a laughingstock. Stories circulate about how he shouts in his mansion to hear echoes, and he writes greeting card jingles. His simple honesty actually wins admirers and the snide news does not hurt him. Business partners of his uncle become incensed when Longfellow sets up a foundation for the poor, and a sanity hearing is scheduled to institutionalize him. Longfellow wants to fight them, but discovers the girl he loves is the spy in his house who is writing the mean-spirited stories for the newspapers. Deeds allows himself to be painted as a pixilated fellow. A series of witnesses testify to his humanitarianism, and it kindles a spark in Longfellow to fight back and to prove his sanity in an insane world.