As the True Cross Academy's grand festival approaches, exorcists and exwires join forces to keep demons at bay. Rin Okumura, along with his brother Yukio and Shiemi Moriyama, is given the task of exorcising the Phantom Train, a demon that devours souls. During the exorcism, the Phantom Train breaks free and wreaks havoc on the academy. Amidst the chaos, Rin comes across a childlike demon he calls Usamaro and strikes up an unusual friendship. However, as Rin delves deeper, he learns that Usamaro's powers might be more malevolent than he initially suspected.