In an era of warring states, the mountain-locked military nation of Tenrei and the vast plains-ruling power of Enra have been locked in conflict for generations.

Guest, raised in a remote village in Tenrei, 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 creed.

After years of grueling training, Guest was granted a single 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, securing victory 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 even once.

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

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

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

Waiting there, Ayano welcomes Guest without attempting to flee.