France, May 1940. Behind the deep Ardennes forest at the Belgian border. Opposing his fellow villagers who fleed before the imminent German invasion, Gustave has decided to stay with his two grandchildren, teenage Camille and young boy Etienne. The elderly man is alarmed by the arrival of a group of stranded soldiers from the French African colonies. But his suspicions are soon overcome by the humanist and whimsical attitudes inspired by a younger generation.