The fake (1953)

Someone is stealing priceless paintings from the great museums of the world and replacing them with nearly flawless forgeries. Leonardo da Vinci's "Madonna and Child" is being shipped to London's Tate Gallery for a special exhibition, and Paul Mitchell is assigned to protect it. Upon the painting's arrival, Paul realizes it has been switched. Eager to collect the museum's $50,000 reward, he teams up with Mary Mason, a Tate employee, to recover the original.