A young woman named Chizuru Yukimura arrives in Kyoto in 1864, disguised as a man, in search of her missing father, a doctor of Western medicine. On her first night in the city, she is attacked by savage, zombie-like creatures with a thirst for blood but is rescued by the Shinsengumi, the shogunate’s special police force. The Shinsengumi take her to their headquarters for questioning, where her true identity is quickly discovered. They reveal they are also looking for her father, who is suspected of creating the Water of Life, a forbidden elixir that transforms those who drink it into a Rasetsu, or Fury, granting them superhuman strength and a healing factor at the cost of their sanity and a shortened lifespan.

To keep an eye on her and prevent the secret of the Water of Life from spreading, the Shinsengumi allow Chizuru to stay with them as a page to the vice-commander, Toshizo Hijikata. As she becomes integrated into the group, she grows close to its many notable members, including the kind but stern commander Isami Kondo, the cheerful and youthful Heisuke Toudou, the quiet and loyal swordsman Hajime Saito, and the prodigy swordsman Souji Okita, who is secretly battling a terminal illness.

The narrative unfolds against the backdrop of the collapse of the Tokugawa shogunate. The Shinsengumi struggle to maintain order in Kyoto while battling both political enemies who support the emperor and the growing threat of the Rasetsu. As the group’s enemies multiply, several members, including Okita and Heisuke, are fatally wounded and forced to drink the Water of Life to survive, becoming the very monsters they fight. The story also reveals that Chizuru herself is not human but an Oni, a descendant of a powerful demon clan, making her a target for other Oni who wish to use her for their own purposes.

The storyline is composed of multiple narrative arcs that follow the different routes of the original source material. One arc focuses on Okita, exploring his struggle with his illness and his eventual transformation into a Rasetsu to protect Chizuru. Another follows Hijikata, depicting his burden as the leader and his final, doomed battles against the imperial army. A third arc centers on Heisuke, who becomes a Rasetsu after being gravely wounded, and his subsequent despair and guilt over losing his humanity. The overarching conflict culminates in the tragic decline of the Shinsengumi as they are overwhelmed by historical forces, leading to the deaths of most of its members in a final, bittersweet conclusion.