A penniless British lord passes a street urchin off as a lady to sell her to a rich husband.

In Houndsditch, London, in the mid-eighteenth century, Kitty, a hungry gamin, is beaten by her keeper, Old Meg, when she fails to bring home expensive buckles. After Kitty is caught stealing a pair of shoes belonging to noted painter Thomas Gainsborough, Gainsborough paints her portrait, titling it "Portrait of an Unknown Lady." Ne'er-do-well Sir Hugh Marcy visits Gainsborough with Brett Harwood, the Earl of Carstairs, and meets Kitty. Hugh, who was expelled from the Foreign Office in favor of the Duke of Malmunster's nephew, and presently is close to debtor's prison, takes Kitty home to be his scullery maid. Gainsborough's work is rivalled only by that of Sir Joshua Reynolds, and at the unveiling of Kitty's portrait, all of London's high society is awed by the anonymous beauty. When the Duke of Malmunster demands to know who she is, Hugh fabricates Kitty's origins so that the duke will want to marry her, and the duke agrees to restore him to the Foreign Office in exchange for an introduction to Kitty. Hugh and his drunken aunt, Lady Susan Dewitt, give Kitty etiquette and elocution lessons. When Hugh is taken to debtor's prison, Kitty, now in love with him, marries her neighbor, an ironmonger named Jonathan Selby, in order to pay for Hugh's release. When Selby catches Kitty stealing from him and starts to beat her, her devoted maid kills Selby, then herself. Kitty soon learns she is pregnant and agrees to marry the duke for Hugh's sake. The duke, an old man who is desperate for an heir, is delighted to learn, at the return of their honeymoon, that his bride is pregnant. She gives birth to a boy, but after his first glimpse of the baby, the duke, who has a weak heart, falls down the stairs and dies, leaving a considerable fortune to his duchess. When Hugh again suggests that Kitty marry, she angrily confesses that she is in love with him, but he cannot see past her sordid upbringing. While out with Gainsborough, Kitty again meets Brett, who has just returned from India, and he falls in love with her. While viewing Gainsborough's latest portrait of Kitty as the Duchess of Malmunster, Hugh finally realizes he is in love with her, but Kitty meets his ardent confession with bitterness and throws him out. At Brett and Kitty's engagement party, Hugh brings Old Meg to meet Brett and divulge Kitty's past, believing that Brett will call off the marriage. Brett is determined to marry Kitty despite her lowly birth, however, causing Hugh to exit graciously. Touched by Hugh's genuine love, Kitty runs after him, and they kiss.