It's the summer of , and Guy and Sarah Masson have traveled with their parents from Paris to a small resort village on Lake Constance, in Austria. What Guy thinks will be an ordinary vacation is transformed into one of discovery and delight when he and Sarah meet a ten-year-old Austrian boy, Willy Schiller. They swim and race their model sailboats in the lake, fight mock duels as knights through the woods, and play chess in their rooms when it rains, while Guy and Sarah plot how Willy can come back to Paris with them in the fall. The three become inseparable that first summer, and each year thereafter their friendship grows -- until the summer of . Suddenly the world is at war, and these best of friends find themselves on opposing sides.