Inspector Bob Deering of New Scotland Yard is surprised to learn that he has inherited property and the title of Earl of Garminster. Bob is more interested in solving crime than being Lord of the Manor and accepts the challenge of finding 'Sparkle', a female jewel thief believed to be staying at the Ambassador Hotel. Bob does not know what Sparkle looks like, only that she has just arrived from Vienna and sometimes poses as a maid.
At the offices of the Daily Journal, Assistant Film Critic Patricia Wayne tells the boss that she wants to do a big story. He sends her to meet former film star Nedda Beaumont, who is staying at the Ambassador Hotel. Patricia and Bob cross paths when they arrive on their different missions. Bob also bumps into old friend Greta Brandt in the hotel foyer; she has just got back from Vienna.
Patricia poses as a maid to get the inside story on Nedda Beaumont. When Nedda goes out for the evening, Bob decides to speak to her maid, suspecting that the former film star could be Sparkle. He catches Patricia trying on one of Nedda's fur coats over her maid's outfit and suggests that she borrows some of Nedda's clothes so they can go out dancing. They enjoy each other's company but when Patricia suddenly runs off at midnight, Bob begins to suspect that she could be Sparkle. When Patricia returns to the Ambassador Hotel, she is caught wearing Nedda's clothes and given the sack.
Nedda's former press agent persuades Patricia to steal a string of Nedda's pearls for a publicity stunt. After a mix-up, Patricia is caught stealing pearls from Bob's friend Greta Brandt by mistake. A chase ensues and Patricia hides inside one of Nedda's trunks which is taken aboard the ocean liner Goliath. Bob is aboard the Goliath in pursuit of Nedda, who he believes to be Sparkle, and he discovers Patricia in the trunk. When they realise that Bob's friend Greta has missed the boat but her luggage is already aboard, Patricia poses as Greta, wearing her clothes and sleeping in her cabin.
Gangster Smiles Hogan, believing that Patricia is Sparkle, tells her that top American jewel thief Ottoman wants to work with her. Taggart, a detective working for an insurance company, intends to arrange for Patricia (who he too believes to be Sparkle) to be arrested in New York. As he has fallen in love with Patricia, Bob ties up Taggart in his cabin to prevent this happening.
The Goliath arrives in New York and Patricia is taken to meet Ottoman in a car full of gangsters. Bob realises the true identity of Sparkle when he sees the name on the envelope of a telegram in the possession of Detective Taggart - it is his friend Greta Brandt. Patricia is taken to a speakeasy and Ottoman tells Patricia his plan to steal jewels that night with her help. The real Sparkle arrives and plots with Ottoman to frame Patricia for their imminent jewel heist.
At the Trocadero nightclub, Patricia is forced to perform a song and dance routine to cause a distraction. In the middle of Patricia's number the lights are dimmed so that Ottoman can steal a diamond necklace. Patricia runs up to a balcony and shines a spotlight on Ottoman in an attempt to foil the robbery. A big shootout ensues. As Patricia struggles with Ottoman on the gantry, Bob comes to her rescue. Ottoman and Sparkle are arrested and Smiles fetches a registrar to marry Patricia and Bob. Patricia thus becomes Duchess of Garminster.
