Boukyaku no Senritsu  

The Melody of Oblivion, 忘却の旋律

In the 20th century, there was a war between humans and monsters. afterward, the monsters took over the world but kept themselves hidden from public view. a young boy named bokka is curious about what happened to the meros warriors who defended humanity against demons during that time. he meets kurofune and learns of their powerful weapons and abilities. the meros are the only ones who can hear a mysterious melody known as melody of oblivion, which is hidden away and awaiting rescue to save mankind. during his journey, bokka encounters various monsters and their agents, allies, and friends as he discovers his newfound powers. he must continue fighting against evil in order to release boukyaku no senritsu and restore the world's forgotten melody.