In Arboral Year 1294, the world is dominated by nine massive towers known as the Spires, ruins of an ancient advanced civilization that hold lost technology and knowledge far beyond present-day craftsmanship. The two greatest Spires are controlled by the Papal States and the Erin Empire, two rival powers locked in an unending war for dominance over the remaining towers and the secrets they contain. Humans born with small branches protruding from their heads, called the Branched, each possess a unique supernatural ability such as invisibility or extraordinary luck. However, these powers only function within a Spire's field of influence, and any Branched who remains outside a Spire's range for too long will die. Some nations openly acknowledge the Branched; others keep their existence hidden.

Luca was raised from infancy among a brutal mercenary band and serves as their bugler, but he despises the life of violence and bloodshed. His one dream is to abandon the battlefield entirely and become a true musician, a maestro. After a battle leaves him for dead, Luca is rescued by the mysterious Pontiff, the leader of the Papal States. The Pontiff reveals that Luca himself is one of the Branched, gifted with the strange power to see the sound of his instrument as visible waves. In exchange for supporting Luca's dream of becoming a maestro, the Pontiff recruits him to lead a special squad of Branched warriors in the war against the Erin Empire. Central to their agreement is an even more dangerous mission: Luca must kill the Garland, a nearly unstoppable Branched whose power is said to be capable of destroying entire nations.

During his first campaign, Luca successfully uses his abilities to conduct a retreat and is made captain of the Branched squad. He forms a friendship with Miura, a fellow Branched, and leads a mission to recapture an abandoned fort occupied by bandits. In their first proper battle together, Luca and his squad intercept an Imperial army at Fort Cannoli. Fellow squad members Zoe and Miura each defeat enemy Branched, securing a victory for the Papal States. But that same night, the Garland appears and annihilates both Fort Cannoli and the surrounding countryside in a devastating display of power. Witnessing the destruction, and also the camaraderie and hope still present among his comrades, Luca begins to reconsider his cynical outlook on life. He becomes determined not only to survive the war but to defeat the Garland and protect the good he has begun to see in the world, all while holding onto his dream of one day putting down his bugle for good and becoming a maestro.