A realm of demons, Adribune. The Overlord who once ruled this world has suddenly vanished. The three nations and the ruling state, four countries in total, seize the chance to start fighting among themselves. In the growing chaos, the Radiant Realm sends its heroes to take control of the Shadow Realm. Amid the turmoil, a lone demon named Kaine washes ashore on a remote island nation. Kaine has lost all memories and remembers only his name and the need to search for fragments of his past. Iroha, who has been caring for Kaine, witnesses the immense power hidden within him. Hoping to save her country from the heroes' advance, she begs Kaine to lend his strength.