Keisuke Sanada is the bodyguard of Hinako, the daughter of a renowned billionaire. He considers Hinako's well-being as the most important aspect in his life. As he was searching a way to lengthen Hinako's lifespan, which is being shortened by an incurable disease, he found himself invited to the magi sacrificing ritual of Ragnarök by Jin Arizuka, where he had the chance of saving Hinako's life. Although unwilling, he agreed to participate in the ritual, which was referred to as a survival game by the Game Master, as the Sleeping Demon (眠り鬼 nemuri oni), hunting down other participants to take their life force and use them to cure Hinako.