On the way to a school trip, Kaito Ise and his classmates fall asleep and awaken in the middle of a death game, where they are pitted against each other until only one survives. However, the event organizers are in for a nasty surprise when Kaito realizes the circumstances—he is an "isekai returner." Under the facade of his youthful face, he has spent a decade in a fantasy land where even the weakest monsters were major threats. After saving the world, Kaito returns to Japan, where only a few seconds have passed since his departure. His only wish is to live a peaceful, mundane life and enjoy his school trip.

Against a man once called the strongest, these organizers are complete small fries. Using his ridiculously overpowered skills, Kaito crushes them all with ease. But as soon as things return to normal, new problems crop up one after another! A peaceful life—is that too much to ask for?