American Paul Mitchell is assigned by a New York art collector to guard his painting of Leonardo da Vinci’s “Madonna and Child” as it travels to an exhibition at London’s Tate Gallery. After the crate supposedly containing the painting is stolen from the London Docks, insurance investigator Smith, whose company has insured the artwork for a million dollars, reports the theft to gallery director Peter Randall and private collector Sir Richard Aldingham, who has sponsored the special exhibition. However, Paul shows up with the painting and explains that the stolen crate was empty and that the canvas traveled in the ship's safe. Paul is hoping to trace two other da Vincis, which were stolen in Florence and New York and replaced with excellent fakes, and claim a fifty-thousand dollar reward. At a reception for the opening of the exhibit, Paul meets a fellow American, Mary Mason, who works in the gallery’s publications department, and her father Henry, an eccentric, frustrated painter of considerable talent. Paul invites Mary to dinner and later ingratiates himself with her father by commissioning him to paint a portrait of her. In order to understand how galleries can be deceived by forgeries of paintings, Paul talks with a Tate expert who explains the tests used to distinguish fakes from their originals. One night, despite the gallery’s excellent security system, a thief enters, takes the “Madonna” and substitutes a fake. Paul chases the thief over the gallery’s roof, but loses him. The Tate’s expert examines the forgery and states that it is the most perfect he has ever seen. Although Paul is puzzled by the thefts of the da Vincis, which would be impossible to fence because they are so well-known, he relentlessly pursues some leads. After discovering that Mason was in Florence and New York around the time the other paintings were stolen, Paul “borrows” one of Mason's originals from Mary and has it analyzed. The expert reports that it contains unique paints, ground in the manner of the old masters. Later, when Paul and Smith find Mason hanged in his studio, Paul suspects that he was murdered. Paul pays a visit to Aldingham’s home to advise him of the latest developments, and after he leaves, Aldingham opens three panels in his study, revealing the stolen paintings. Weston, the thief who stole the artworks at Aldingham's behest, reminds his employer that he is the only person who knows that Aldingham commissioned Mason to make fakes of the originals and that Mason was unaware of their purpose. Weston also warns him that Mary may eventually find out that her father was involved. After Weston rejects Aldingham’s suggestion to kill Mary, Aldingham gives him a poisoned drink that proves fatal. Aldingham then phones the distraught Mary, offers to drive her to his sister’s country place where she can rest, and arranges to pick her up. Meanwhile, as Paul and Smith search for clues at Mason's studio, Paul discovers a rough painting of Aldingham’s study that shows the panels open and the paintings inside. Paul sends Smith to inform the police, then goes to Aldingham’s house where he surprises Aldingham and Mary when they arrive. When Paul confronts Aldingham with the paintings, Aldingham states that they are simply copies he commissioned from Mason. However, when Paul threatens to throw destructive acid on one of the “copies,” Aldingham draws a gun and orders him to back away from the painting. Although Mary knocks the gun out of Aldingham’s hand, Paul still throws the acid, ruining the painting. Paul then explains that the destroyed painting was actually the fake, as he and Randall had earlier switched the paintings. Paul produces the true original, observing that Aldingham fell for his own scheme.