Judy Howard, secretary and niece to Joshua Howard, a wealthy diamond merchant and art collector, returns to her Mayfair home after a shopping trip. Unable to find her key, she rings her doorbell, and is surprised to find an unfamiliar face opening the door. Perkins, the butler, introduces Richard, who has applied for the post of footman. He has agreed to employ him, subject to the approval of the formidable chief housekeeper, Mrs O'Malley, who is in charge of all 'below stairs' staff, including Perkins and Rosie, the flirty young kitchen maid. Mrs O'Malley approves of Richard when he declares an enthusiasm for the game of bridge. When Richard furtively arrives for the weekly 'below stairs' game, Perkins, to Richard's annoyance, addresses him as 'my lord'. Judy finds Richard a most unlikely footman, who does not 'know his place', lacking the expected deference. He wears an old Etonian tie, and is knowledgeable about paintings and classical music.
In a nightclub, 'spiv' Bacon is trying to sell a forged painting to gullible collectors. He arrives at the Howard house with his 'hard sell' and a letter from Judy's uncle asking for a cheque to be handed to him. Judy is suspicious, and asks Richard for a second opinion. He says the picture is a fake, and escorts Bacon from the premises.
Judy has organised a dinner party marking her uncle's return from Cape Town. Enquiring about the painting, he is annoyed to hear that the seller was sent packing. He blames Richard, but Judy defends him. Joshua later explains that he knew Bacon was crooked but wanted to use the cheque as a trap to trace the theft of his Gauguin.
Joshua is also to meet Judy's two current suitors - the vain film star Basil Maitland and George, a Marquis struggling financially at his country seat, Borechester Towers. At dinner, suitors Basil and Borechester are seated either side of Judy. Borechester mentions his brother Richard, missing for 10 days with family assets. He plans to engage a private detective to track him down. Richard, serving, skillfully avoids being recognised by his brother.
Judy dances with Basil, but Richard has arranged for Rosie to phone him, posing as 'a fan': a call Basil cannot resist. Richard can now dance with Judy. At end of the evening, the guests say farewell, and Richard (downstairs, washing the dishes) and Judy (upstairs) imagine dancing romantically together.
On his afternoon off, Judy joins Richard on the bank of the Serpentine. She asks him about his background, but he is evasive. They visit a nightclub, where Richard challenges one of the art thieves. But Judy spots Borechester at an adjoining table, and they make a quick exit to the Lyceum Ballroom. Rosie, in the Lyceum balcony, is chatted up by the private eye who is employed to trace Richard. Hurt to see Richard and Judy dancing below, she confirms the identification. The MC introduces Basil Maitland on stage and Richard and Judy flee. Outside Judy's house, the detective sees Richard (after kissing Judy) go below stairs.
Borechester arrives and tells Judy that her 'footman' is his errant brother. Richard has meanwhile gone to London airport, leaving a note that the missing Gauguin is with a crooked dealer. Joshua recovers his picture, and the dealer is arrested. Judy finds Richard at the airport and he relates his story: he sold the family pictures in America, but received a telegram saying he had been given a dud cheque. Leaving the ship in hiding to avoid his waiting brother, he wandered the streets. At London Zoo, he met Perkins, a former retainer and now the Howards' butler, and asked if he could lay low for a time. Back at the house, Perkins explained there was a footman vacancy, but it required experience. When the doorbell rang, Richard seized the initiative and dashed upstairs to answer it. It was Judy.
Richard returns home with Judy. He takes another envelope - unopened - from his pocket. It is a telegram saying that the cheque was good after all. Richard is in the clear, and Joshua is delighted that his Gauguin is safe. Judy tells Joshua that she and Richard are to marry.
