One day, a middle-aged man who unexpectedly reincarnates into another world ponders an idea. An idle game. In the busy modern society, it is a game system that allows one to easily and quickly become strong. But what if this game theory were to be brought into another world? This is a story about a young boy who aims to create an “efficient idle game” while struggling with the differences between games and reality. Unbeknownst to him (and even himself), he ends up saving the world from crisis and even starts playing an idle game against the Demon Lord.