According to legend in Japan's Meiji Era, the "Orochi," an evil giant snake spirit, returned every 300 years to cause destruction. To defeat the Orochi, an elite fighting force known as "Yamainu" was created. However, Yamainu's leader, Sousei Abeno, and brothers Soramaru and Chuutarou Kumou become entangled in a secret government department's unethical experimentation with Orochi cells. The brothers discovered several dying escapees of the experiments and Soramaru meets Ageha, a criminal doctor allied with a test subject named Tora who killed cell project researchers using abilities from compatible Orochi cells. Despite Ageha and Tora's crimes, Soramaru chooses to side with them due to his opposition to the exploitation of Orochi cells. This disagreement results in a conflict between Sousei and Soramaru over the ethics of using the immortal enemy's essence to create new drugs, which may have to be resolved through combat. However, Soramaru has never defeated Yamainu's captain.
