After a fight with his foster father Shirou Fujimoto, Rin Okumura's life is transformed overnight when Rin is revealed to be the son of the demon lord Satan, king of the demonic realm of Gehenna. Soon after, his foster father becomes possessed and dies at the hands of Satan, leaving Rin and his twin brother Yukio alone in the world. Seeking revenge for Fujimoto's death, Rin attends True Cross Academy with Yukio in order to become an exorcist and join the Knights of the True Cross. Rin's new path, however, is filled with many difficulties. While new classes and dealing with his peers are already troublesome enough, he is also forced to conceal his newfound demonic powers and parentage. In addition, plenty of evildoers want to use him for their own ends. One misstep could lead to disaster, as not everyone at True Cross Academy is exactly who they seem to be...