Whilst sightseeing in 1979 Paris, the Doctor and Romana notice a series of unexplained temporal disturbances. When a visit to the Louvre lands them in hot water, they realize that a certain Count Scarlioni knows more about advanced technology than a twentieth-century Parisian should. With British detective Duggan in tow, the time travelers become embroiled in an audacious plot to steal the Mona Lisa and sell it on the open market. Not only that, but Count Scarlioni appears to have more da Vinci masterpieces at his disposaleach one apparently genuine. What time experiment is Scarlioni forcing a genius professor to undertake? What is behind his suave persona, and what are his plans for not only Paris but the whole world? The Doctor and his friends must discover the answers before time itself runs out.