Following a heated altercation with his guardian, Shirou Fujimoto, Rin Okumura's world shifts dramatically when he learns of his lineage as the son of Satan, ruler of Gehenna. Tragedy strikes as Fujimoto is possessed and perishes by Satan's hand, leaving Rin and his twin brother Yukio orphaned. Driven by a thirst for vengeance, Rin enrolls at True Cross Academy alongside Yukio to master exorcism and enlist in the elite Knights of the True Cross. However, Rin's journey is fraught with challenges. Balancing academic rigors, social dynamics, and the clandestine nature of his demonic heritage proves arduous. Amidst the struggle, malevolent forces seek to exploit Rin for their sinister agendas. The academy harbors secrets, and trust is a fragile commodity, for not everyone is as they appear in this treacherous realm.