During the first attempt at building the Channel tunnel between England and France, Miss Seeton makes a crossing of her own. She and a group of Plummergen schoolchildren take a day trip to France to explore World War II battlegrounds. Art-lover Miss Seeton suggests a side-trip to the Chateau de Balivernes. There Monet's celebrated garden is being re-created by the Comte de Balivernes, who turns out to be a war-time ally of Plummergen's own Sir George Colveden. The Comte's reciprocal visit to Plummergen is marred when a woman is found murdered. Miss Seeton is asked to sketch a likeness for the police. Meanwhile, a large WWII bomb is discovered blocking the path of the proposed Channel tunnel. For obvious public relations reasons, the news is kept top secret, even though a charity bulldozer; race is scheduled at the sight. During the race, the police make a shocking discovery in Miss Seeton's trusty sketch pad - one that leads to an explosive conclusion...