Seinoshin Aizawa is introduced as a comrade of Touma Kusaka within the Chōshū clan during the 1862 Expulsion of Foreigners movement. He joins Kusaka in the attempted arson of the British Embassy, and both escape Bakufu authorities with Keiichirou Akizuki's aid.

Later, Aizawa observes Kusaka and Akizuki meeting secretly to study English. This discovery creates tension due to their opposing clan loyalties, forcing Kusaka to end the meetings to avoid conflict.

During the Boshin War, Aizawa fights for the Imperial faction. Four years after the war, he serves as an officer in the new Meiji government's law enforcement. His investigation uncovers Akizuki concealed within Kusaka’s residence.

Aizawa confronts Akizuki directly in his hiding place, stressing the dishonor of his survival while former Bakufu comrades remain imprisoned. He leaves a tantō with Akizuki, implicitly urging seppuku to regain dignity.

Aizawa subsequently returns to Kusaka’s property leading a police contingent. While Kusaka confronts him, Akizuki uses the provided tantō to end his life at a nearby four-trunked tree. Aizawa’s actions trigger this tragic conclusion for both protagonists.