It's 1920 France and Mathilde (AUDREY TAUTOU) has just received word that her boyfriend and childhood friend Manech (GASPARD ULLIEL), perished in WWI battling the Germans. A young woman who's lived with her aunt Bénédicte (CHANTAL NEUWIRTH) and uncle Sylvain (DOMINIQUE PINON) ever since her parents' deaths some time ago, Mathilde refuses to believe the news to be true. After all, she's sure she would feel it in her heart if he were gone from her life.
Accordingly, she sets out, along with the aid of detective Germain Pire (TICKY HOLGADO), to discover what happened to Manech and determine his fate. She learns that in 1917 he and four other soldiers -- Six-Sous (DENIS LAVANT), Ange Bassignano (DOMINIQUE BETTENFELD), Benoît Notre Dame (CLOVIS CORNILLAC) and Kleber Bouquet, a.k.a. the Eskimo -- were accused of self-inflicting wounds that presumably would be reason to be sent back home. Instead, the men were court-martialed and sentenced to abandonment in the ravaged, no man's land between the Germans and French on the front lines.
As Mathilde and Pire continue their independent investigations and a distraught woman, Tina Lombardi (MARION COTILLARD), seeks revenge on those she believes responsible for her boyfriend's death, young Mathilde remains resolute that not only will she find that Manech is alive and well, but also that they'll ultimately be reunited.