Ordinary college student Nuowei accidentally transmigrated to another world, and was chosen as the hero to subjugate the Demon King. But when he woke up again, he had lost key memories and was trapped deep inside the Demon King's Castle. Even more bizarrely, the yandere Demon King Weishilin not only knows him, but also insists that the two had an unforgettable past. In order to recover his memories and freedom, Nuowei matches wits and courage with her while secretly searching for a way to escape the Demon King's Castle, yet gradually discovers that the truth he forgot seems to be related to Weishilin.