Itsuki, a boy who was murdered by a random street attacker, is reincarnated. He finds himself in a Japan where man-eating monsters known as 'demon devourers' exist. And to top it off, he's been born into a family of exorcists who use magic to exterminate these monsters! The children of exorcists are prime targets for Demons. If he's weak, he will die again. So Itsuki makes the decision to become strong so he never has to die again. Aiming to be the strongest, the starts to practice magic from the time he's a baby... until he eventually realized something. "Crap... I trained too hard..." Working so hard he became the strongest though being a commoner?! The curtain rises on an effort-made-overpowered fantasy, where our hero uses power beyond all limits to defeat demons one after another.