After the liberation of Paris, officials at the Louvre Museum make preparations to retrieve the Mona Lisa, which was sent to the National Art Gallery in London for safekeeping during World War II. Sir James Collison, the National Gallery's director, is in charge of the painting, as well as other treasures that were hidden during the war, and has just been awarded a peerage for his years of service to the Crown. While Collison and his secretary Tony, who is also his granddaughter, finalize their plans to turn over the Mona Lisa, art collector Carl Hoffmeyer enacts his scheme to steal the painting. Hoffmeyer, who is known as the "Raffles of the art world," has stolen many artworks, which he keeps secreted in his house for his own enjoyment. Henri and Jules, Hoffmeyer's minions, convince Collison that they are the Louvre officials and get the painting from its hiding place inside a hollowed-out mountain. When Hoffmeyer examines the painting, however, he realizes that it is a copy. Curious about Collison's part in the fraud, Hoffmeyer goes to his house and coolly admits to the theft. Collison's astonishment at Hoffmeyer's assertion that the painting is a forgery convinces the thief that Collison was also duped, and he turns his attention to Sam Todworthy, a crooked antiques dealer who had warned him earlier that the Mona Lisa was not located where he thought. Before Hoffmeyer arrives at the shop owned by Sam and his wife Emma, the couple is confronted by Anton Miran, the brother of the French official who originally brought the Mona Lisa to London. Miran, who snuck the painting out to the Todworthys, demands that Sam return it, but Sam refuses and shoots him. After disposing of Miran's body, Sam greets the just-arrived Hoffmeyer and demands £100,000 for the painting. Hoffmeyer scoffs at Sam's arrogance and leaves, then orders Henri and Jules to kill Sam. When Jules refuses, Hoffmeyer stabs him with his sword cane. Soon after, Jules's body is discovered and Inspector Bob Cartwright of Scotland Yard begins an investigation. The next day, Collison is confronted by Sam, who shows him the Mona Lisa and offers to sell it to him. The troubled Collison discusses the situation with Tony, who advises him to contact Scotland Yard, but Collison refuses, for he is afraid that the resulting scandal might endanger the health of his invalid wife Mary. Collison also feels responsible for the Mona Lisa and determines to raise the money for its retrieval. After Collison leaves the office, Tony receives a call from Professor Renault, the director of the Louvre, who warns her that his two assistants were kidnapped, and that she should give the Mona Lisa only to him. Hoping to solve the case, Tony consults Bob, who is her fiancé, and apprises him of Collison's predicament. Convinced that Sam will destroy the painting rather than go to prison, Collison refuses to tell Bob his name and is shocked when Bob receives word that Sam has been murdered. Knowing that Hoffmeyer must be involved, Bob and Collison interview him, but Hoffmeyer, who has hidden the sword cane that killed both Jules and Sam, denies any culpability. Later, Collison returns to Hoffmeyer's house in an attempt to break in and retrieve the painting, but Hoffmeyer catches him and, holding him at bay with the sword cane, takes him to the study. Before Hoffmeyer can kill Collison, he is shot by Emma, who is in turn run through by Hoffmeyer's sword. The dying Hoffmeyer then opens the secret panel hiding the Mona Lisa but is prevented from slashing it by Collison. Tony and Bob arrive and quickly take the painting to the airport, where they quietly exchange it with the fake one, which they had given to the departing Renault. Years later, Collison relates the story of his adventure to Tony and Bob's young son, who admires the fake Mona Lisa hanging on their wall.