William is a cat who is called in to investigate the theft of a masterpiece from a gallery in Paris. He quickly spots a suspicious character, a figure in a trench coat, hat and dark glasses, and pursues him throughout the city. Eventually the thief is exposed and the painting restored, thanks to William.

This wonderfully colourful, funny story invites the reader to search with William for the stolen painting, the 'Mona Cheesa' (a clue to the thief is in the title). We scour pictures of the Parisian streets and join with him in his delight at uncovering the perpetrator(s).