In 1916, during World War I, a German and a French fighter ace by the names of Gunther von Beckman (Hoffman) and Jo Cavalier (Belmondo) manage to drag each other out of the sky.

An argument and subsequent fistfight about who is to be whose prisoner is interrupted by an artillery barrage on their position, forcing both to work together in order to survive.

In a humorous side scene, Corporal Adolf Hitler (Meisner) is berated by his frustrated First Lieutenant Rosenblum for his clumsiness.

Arriving there, Jo gets into a fight with Gestapo agents who are demolishing the place, and subsequently he is asked by the whole Rosenblum family to hide them.

The next morning, just before they depart for the stadium for the opening ceremony, Jo re-encounters his old friend Gunther, now a general of the Luftwaffe.

The Gestapo is hot on Jo's heels, a bear drives him and Simon from their forest camp, and they temporarily pick up its cub, whom they spontaneously name Beethoven.