In the 20th century, humans battled monsters but were ultimately defeated. The monsters now rule through fear while keeping themselves hidden. Bokka cherishes memories of the brave Meros Warriors who once protected humanity from the ferocious demons. One day, he meets Kurofune who introduces him to the Meros and their I-bar machines, which were key in the monster battles. Only warriors can hear the Melody of Oblivion, a mystical girl longing to be rescued and aid humanity. Bokka is amazed when he too hears the melody during Kurofune's battle against a demon. Along the way, he forms friendships with monsters and their allies, discovering his own extraordinary abilities. Bokka is on a mission to fight evil and restore the world's forgotten melody, Boukyaku no Senritsu. He sets out on a journey to champion this cause.