Itsuki, a young boy who died at the hands of a random attacker, has been reincarnated. He awakens in a Japan where monsters known as 'Ma' (Demons) roam and attack humans—and he has been born into a clan of 'Exorcists' who use magic to exterminate them! The children of Exorcists are easily targeted by these monsters, and those who are weak will only be killed again. Thus, Itsuki makes a resolve: he will become strong, so that he never has to face death a second time. Aiming to be the strongest, he has been practicing magic since he was an infant... until he realized something. 'Oh no... I worked too hard...' Through sheer over-effort, an ordinary boy has accidentally become the strongest?! The story of a 'hard-work-overpowered' fantasy begins, as he crushes every demon in his path with off-the-charts power!