In an era of warring states, the mountainous military nation of Tenrei and the powerful Enra, rulers of the vast plains, have been locked in conflict for years.

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

After long years of training, Guest was granted a katana by Ayano and recognized as a full-fledged swordsman. Told "I have nothing left to teach you," Guest left his master's side and enlisted in the Tenrei Army.

Eventually, a great war broke out between Tenrei and Enra. Guest achieved numerous feats on the battlefield, overcoming fierce battles alongside his comrades. General Katsutsugu Kurosaki, who led the Tenrei forces, was hailed as a hero of victory, and the war ended with Tenrei's triumph.

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 pursued her as a traitor. Sogen Mikage, the supreme head of military affairs, ordered Guest to subjugate her.

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

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

Waiting there was Ayano, who greeted Guest without attempting to flee.