An era of warring states. The military nation of Tenrei, nestled among the mountains, and the powerful Enra, rulers of the vast plains, have been locked in conflict for generations.

Raised in a remote village in Tenrei, Guest was taken in as a child by the swordswoman Ayano Shirasagi and became her disciple. Ayano was a taciturn and strict mentor, but she never sought strength for its own sake. "A sword is not held to kill people. It is held so that you may remember exactly what you have cut until the very end"—that was Ayano's teaching.

After years of training, Guest was granted a katana by Ayano and recognized as a full-fledged warrior. Told that "there is nothing left for me to teach you," Guest left his master's side to join the Tenrei army.

Eventually, a great war broke out between Tenrei and Enra. Guest achieved numerous feats on the battlefield, securing victory alongside his comrades. General Katsutsugu Kurosaki, leader of the Tenrei forces, was hailed as a hero of victory, and the war ended in Tenrei's favor.

However, just one week later—Katsutsugu Kurosaki was murdered by an unknown assailant.

The culprit was Ayano, who had not participated in the war at all.

Ayano fled, and Tenrei branded her a traitor. Sogen Mikage, the supreme head of military affairs, orders Guest to subjugate Ayano.

"You know her blade better than anyone. There is no one more suited for this task."

Kill his master, and he will be treated as a hero. Refuse, and Guest himself will be executed for treason. After much inner conflict, Guest heads to the mountains to strike down his mentor.

Waiting there is Ayano, who greets Guest without attempting to flee.