In the Monster Soul world, a war was waged between humans and monsters, with the monsters falling on the losing end of a treaty. The peace between the monsters and humans is tenuous and monsters are frequently persecuted by humans. As a result, monsters tend to stay away from humans and keep a low profile. In this earlier work from Hiro Mashima, the author of the popular series Fairy Tail, Mashima creates an action adventure story emphasising the importance of friendship with a light but humorous tone throughout.