The protagonist, Cid, is reincarnated from modern Japan into a medieval European-style fantasy world where magic exists. It is a world of survival of the fittest, where physical enhancement through mana and swordsmanship are highly developed. The Cult of Diablos is a secret organization that manipulates the world from the shadows, plotting the resurrection of the demon Diablos. While aiming to destroy the Cult, Cid himself mistakenly believes that everyone is just playing along with the chuunibyou fantasies he made up. Mana Overload is a phenomenon where one loses control of their mana, causing abnormalities in the mind and body. Possession is a rotting of the flesh that occurs primarily in women with high mana aptitude, such as Elves and Beastkin. In reality, it is a form of Mana Overload; those cured by Shadow who gained superhuman powers became the members of Shadow Garden.