Jun Tendō, a shy and reserved middle school student, bears a distinctive birthmark on his left shoulder signifying his descent from the Magatama clan. This lineage grants him the unique power to awaken and control the mighty eight-headed dragon, Yamata no Orochi.

Suddenly transported 1200 years into Japan's Heian past, Jun arrives amidst a brutal war between humans and Oni. The humans immediately declare him their prophesied savior, a role that bewilders him due to his perceived inadequacies and lack of combat skills. His initial weakness and unassuming nature make him a target for bullying.

His journey is fraught with conflict and moral ambiguity. Jun forms a bond with Orochi; the dragon responds to his emotional state and desires, acting as both protector and manifestation of his growing power. This connection allows Jun to instinctively summon Orochi in moments of crisis, like an attack on his temple refuge. A pivotal shift occurs upon meeting Mizuha, a member of the Oni. She reveals the Oni are not demons, but marginalized people using masks for survival after human nobles colonized their homeland. This forces Jun to question the righteousness of either side, leaving him deeply conflicted.

Jun faces escalating challenges. He discovers the betrayal of Monk Gen'un, who orchestrates the conflict for personal power, including assassinating a temple councillor and transforming warriors like Raiko Minamoto into mind-controlled Four Heavenly Kings. Confronting Gen'un's treachery transforms Jun's uncertainty into resolve. He fully accepts his role, wielding a merged sword forged from a gem holding his latent power and a dagger linked to Raiko's past. In the climactic battle, Jun harnesses Orochi's ultimate form and water-controlling abilities to defeat Gen'un.

This experience changes him deeply. Returning to his original timeline, Jun exhibits newfound confidence and maturity, ready to move forward without his earlier insecurities.