The Doctor and Romana are enjoying a holiday in Paris, 1979, when they become aware of a fracture in time. During a visit to the Louvre to see da Vinci's Mona Lisa, the Doctor purloins from a stranger, Countess Scarlioni, a bracelet that is actually an alien scanner device.
He, Romana and a private detective named Duggan are then 'invited' to the chateau home of Count Scarlioni...