Longfellow Deeds (Cooper) has never been out of Mandrake Falls in his life. He's happy being tuba-player in the town band and writing jingles for greeting cards. But one day lawyers descend on the sleepy town to inform Longfellow that he is a multimillionaire - an uncle has died and left him twenty million dollars.

Deeds agrees to go to New York ("I'd kinda like to see Grant's Tomb.") His new lawyer, Mr. Cedar (Douglas Dumbrille), is confident that he can soon gain power of attorney, boasting to his pals that Longfellow Deeds is as naive as a child. And, true, Deeds interrupts a meeting to chase a fire engine, slides down bannisters, and engages his puzzled butler and valet in an echo contest in the great hall of his new mansion.

However, when Cedar's push comes to shove, he finds that Longfellow is not as naive as he first seemed. "Suppose you get the books straightened out quick so I can have a look at them," Deeds tells the lawyer.

Longfellow Deeds and his fortune are news, and the editor of a big newspaper (George Bancroft) assigns the story to his ace reporter, hardboiled Babe Bennett (Jean Arthur). In order to get the 'real scoop', Babe poses as Mary Dawson, an unemployed stenographer. She contrives to meet Deeds (with hidden photographers in tow, of course) and gains his interest and sympathy.

Deeds is open and friendly with everyone he meets, but he soon finds that each and every one is either condescending or greedy, if not downright hostile. He believes it is only "Mary" he can trust. He sees her every chance he can get, unwittingly providing her with all the exclusive story items she could want.

Her articles begin appearing, dubbing Deeds the "Cinderella Man." The stories delight in poking fun at this 'hick' and his antics: "At two o'clock this morning Mr. Deeds tied up traffic while he fed a bag of doughnuts to a horse. When asked why he was doing it, he replied, 'I just wanted to see how many doughnuts this horse would eat before he asked for a cup of coffee.'!"