Private Matthew Petticrew arrives in France as part of the American Expeditionary Forces, an arrival that a war-weary France desperately hopes will help to end the turmoil. Having faced unthinkable things on the front, he is captivated by the sound of a lullaby, sung by a voice so pure he knows he must have imagined it. But rumors sweep through the trenches like wildfire, dubbing the voice "The Angel of Argonne", a mysterious presence who leaves behind wreaths on unmarked graves and footprints in the war-pocked soil.

Raised wild in the depths of the Forest of Argonne, France, Mireilles finds her world rocked when war comes crashing into the idyllic home she has always known, taking much from her. When Matthew discovers Mireilles, three things are clear: She is alone in the world, she cannot stay, and he and his two unlikely companions might be the only ones who can get her to safety.