In an alternate world where Yokai exist, there are special humans who are able to see them; and Mio Kojima is one of those unlucky few. At fifteen years of age, he's just started his first year of high school and is looking forward to spending a relaxing summer with his childhood friend, Sayuri Izumi. However, when a monstrous entity known as the Mononoke almost brings him to death's door, he is saved by his grandfather who belongs to a group known as Exorcists; specially trained humans capable of fighting these evil spirits. With Sayuri having awakened her spiritual powers and getting accepted in a prestigious school for young Exorcists, Mio strives to find a way to stay by Sayuri's side while fulfilling his promise to protect her. His search leads him to a peculiar company offering a solution to his predicament. Though his wish is granted, trickery is afoot and a mix-up results in an irreversible transformation of his body. Despite this change, Mio remains steadfast in his commitment to Sayuri and embarks on a new journey alongside her, which promises to not only shape his destiny as an Exorcist and as an individual, but also has the potential to alter the fate of the world itself.