During the Great Depression, Godfrey "Smith" (William Powell) lives alongside other men down on their luck in the city dump. One day, snooty socialite Cornelia Bullock (Gail Patrick) offers him five dollars to be her "forgotten man" for a scavenger hunt. Annoyed, he backs her up until she falls on a pile of ashes. She leaves in a fury, much to the glee of her sister Irene (Carole Lombard). After talking with her, Godfrey finds her to be kind, if a bit scatter-brained. He offers to go with Irene to help her beat Cornelia.
In the ballroom of the Waldorf-Ritz Hotel, Irene's long-suffering businessman father, Alexander Bullock (Eugene Pallette), waits resignedly as his ditsy wife, Angelica (Alice Brady), and her mooching "protégé" Carlo (Mischa Auer) play the frivolous game. Godfrey arrives and is "authenticated" by the scavenger hunt judge as a "forgotten man". He then addresses the idle rich, expressing his contempt for their antics. Irene is apologetic and offers him a job as the family butler, which he gratefully accepts.
The next morning, Godfrey is shown what to do by the sardonic, wise-cracking maid, Molly (Jean Dixon), the only servant who has been able to put up with the antics of the family. She warns him that he is just the latest in a long line of butlers. Only slightly daunted, he proves to be surprisingly competent, although Cornelia still holds a sizeable grudge. On the other hand, Irene considers Godfrey to be her protégé, and is thrilled by his success.
A complication arises when a guest, Tommy Gray (Alan Mowbray), greets Godfrey familiarly as an old friend. Godfrey quickly ad-libs that he was Tommy's valet at school. Tommy plays along, mentioning Godfrey's non-existent wife and five children. Dismayed, Irene impulsively announces her engagement to the surprised Charlie Van Rumple (Grady Sutton), but she soon breaks down in tears and flees after being politely congratulated by Godfrey.
Over lunch the next day, Tommy is curious to know what one of the elite "Parkes of Boston" is doing as a servant. Godfrey explains that a broken love affair had left him considering suicide, but the optimistic, undaunted attitude of the men living at the dump rekindled his spirit.
Meanwhile, when everything she does to make Godfrey's life miserable fails, Cornelia sneaks into his room and plants her pearl necklace under his mattress. She then calls the police to report her "missing" jewelry. To Cornelia's surprise, the pearls do not turn up, even when she suggests they check Godfrey's bed. Mr. Bullock realizes his daughter has orchestrated the whole thing and sees the policemen out.
The Bullocks then send their daughters off to Europe because Irene is getting too attached to Godfrey. When they return, Cornelia insinuates that Godfrey is actually attracted to her, and not Irene. The alarmed Irene stages a fainting spell and falls into his arms. In one of the film's best-known sequences, Godfrey carries the swooning girl to her bed. While searching for smelling salts, he realizes she's faking when he sees her (in a mirror) sit up briefly. In revenge, he puts her in the shower and turns on the cold water full blast. Far from quenching her attraction, this act merely confirms what she had hoped: "Oh Godfrey, now I know you love me...You do or you wouldn't have lost your temper."
When confronted by the rest of the family, Godfrey quits. But Mr. Bullock has more pressing concerns. He first has a private "little chat" with Carlo, throwing the freeloader out. He then announces that his business is in dire financial straits, with both a sinking stock price and his own desperate actions weighing on him. Godfrey interrupts, explaining that he understood some time ago that Mr. Bullock's interests were weakening and sold short, using money raised by pawning Cornelia's necklace. He's been making money while Mr. Bullock has been losing, and has endorsed his gains over to Mr. Bullock's name. He then turns over a bundle of stock certificates to the stunned Mr. Bullock. He also returns the necklace to a humbled Cornelia, who apologizes for her attempt to frame him with the pearls. Godfrey says his goodbyes to all but Irene, and takes his leave.
With the rest of his stock profits and reluctant business partner Tommy Gray's backing, Godfrey had built a fashionable nightclub at the dump, "...giving food and shelter to fifty people in the winter, and giving them employment in the summer." Irene tracks him down and bulldozes the reluctant Godfrey into marriage, saying: "Stand still, Godfrey, it'll all be over in a minute."