During a summer seaside school trip, the Demon Boy arrives at the ocean. Just as he and the Twelve Apostles are enjoying a brief vacation mood, he is mysteriously abducted along with Mephisto II by a strange ray of light. The two are taken to Horai Island, home to the wise sages known as the Eight Immortals. The Eight Immortals urge the Demon Boy to abandon his powers and return to being an ordinary child. They claim that the conflict between humans and demons persists precisely because of his existence. However, the Demon Boy cannot give up his dream of creating a world where both humans and demons can live in peace. He refuses their offer, but the Sphinx—a being that pacifies the souls of this world—begins moving toward the demonic realm. If this continues, demons and mummies worldwide will awaken and start attacking humans. Though the Eight Immortals halve his powers, the Demon Boy uses Solomon’s Flute to mobilize the invisible school and pursue the Sphinx. Yet before him appears a formidable enemy—the Black Demon Satan, who plots to establish a demon kingdom!