France, 1940. German troops have just invaded the country, but in the rural idyll of the family chateau inhabited by 20-year-old Mellie with her impoverished aristocrat father Alexandre and adolescent cousin Robinson, the war still seems far away, apart from the fact that Mellie's rich and unpopular fiancé André has been drafted into the army. For Robinson and his friends the same age, including the temperamental Bertrand, the war is merely a game they play in the remoteness of the blue island in the lake.