Léon Lécuyer, an idealistic history student, manages to escape from the camp where he was imprisoned and comes back to Paris. He hides in his mother's apartment when the Germans, informed by an anonymous letter, storm their block. He runs away once more and leaves for Lyon. Wishing to serve his country, Léon decides to kill Pierre Laval. But he does so badly that he is arrested and condemned to ten years in prison. Meanwhile in Paris, Charles-Hubert and Julie Poissonnard, the owners of the dairy shop "Au Bon Beurre", where Léon's mother shops, thrive by speculating on people's misery, by getting supplies on the black market, by selling goods ten times what they are worth, while blatantly supporting the policies of Maréchal Pétain. But when Charles-Hubert senses the wind turning, he changes attitudes. He saves a Jew and even "organizes" the Resistance in his neighborhood. After the war, the Poissonards, richer than ever, have gained a new respectability. As for Léon, now a history teacher at Lycée Jean-Baptiste Say, he is transferred against his will for ... giving bad grades to Henri, Poissonard's son.