The protagonist Okumura Rin is a student at the True Cross Academy. Although he is the descendant of Flame on the surface, he is actually the son of the demon Satan. He can only suppress this power with the sword of exorcism. After Father Fujimoto was killed by Satan, he found Mephisto Ferris according to Father Fujimoto's last wish and knocked on the door of the Knights of the True Cross, determined to become a demon slayer. His goal is to defeat Satan and beat him up.