1955. Kitaro's father, Medama Oyaji, comes to Yagura Village in search of his missing wife. The village was ruled by the Ryuga clan, which controls Japan's political and business circles behind the scenes. Mizuki, who works at a blood bank, visits the village with a secret mission to mourn the death of the head of his family, and meets Kitaro's father. While an ugly fight unfolds over the succession of the head of the family, a member of the family is brutally murdered at the village shrine. It was the beginning of a terrifying chain of events.

If I am promised to the princess, then I will fight for her sake.

In 1939 C.E., the imperialist nation of Germania invaded a neighboring country. All at once, that war spread throughout Europe, and the era was dragged into a spiral of a great war.

Then, in 1940, Germania's attack turned towards the Principality of Eylstadt, a small Alps country abundant with beautiful water and greenery.

On the outskirts of the country of Helvetia rests the tranquil town of Seize. Upon its cobbled streets, citizens go about their daily lives, undisturbed by the increasingly tense military relations between Helvetia and the neighboring Roman Empire.

It is under these circumstances that the 1121st platoon of the Helvetian army, stationed at the Clocktower Fortress in Seize, receives a new recruit in the young and spirited Kanata Sorami. Having joined the military to fulfill her dream of learning to play the bugle, she excitedly accepts the tutelage of the Sergeant Major, Rio Kazumiya, who happens to be a skilled trumpeter. Working alongside them are the aloof mechanic, Noël Kannagi, the feisty gunner, Kureha Suminoya, and the compassionate Captain Felicia Heideman; together, they experience the beauty of life in Seize and the lasting joy of a community that has persevered in spite of the crumbling world around them.