After a long-standing battle between the demons and humanity, the Demon King Luruslia Nord was laid to rest at the hands of a hero. However, his soul continued to wander. After thousands of years, Luru awakened in a world devoid of demons. Moreover, he was reincarnated as a weak human! To search for his disappeared brethren and to understand the meaning of the new "life" he's been granted, Luru sets forth on a new adventure.

When the Demon Tribes led by a Demon King appeared and launched a massive war at mankind, all hope seems to be lost, until a brave group of heroes managed to turn the tide and even defeated the Demon King. John Serieux, a mid-ranked soldier of World's Resistance against the demon's army, had fortunately been able to witness the Demon King's fall by the Heroes, but also was unfortunately killed a moment after. Miraculously, he was still able to open his eyes, only to realize that his mind had been sent back to the past when he's still a baby! With all the memories of his adult years, the knowledge of many scientific breakthroughs during the war, and information of the upcoming great disaster all intact, John is determined to cultivate his potential efficiently and optimally to help change the future for the best.