While following Kurofune, Bocca and Sayako encounter a dead end, but finds 'Nezumikoukoku', or 'Pyramid Scheme Valley', and also find a dam with a mural of a tear on it. The duo goes to it, and find a beautiful boy with sky blue hair. They also meet up with the artist of the mural, Eichi Hikoyama, who seems to be hired by a woman named Miri Kanaya who owns the dam. Miri also owns mice that gather tears around the world for her dam. Bocca and Sayako pass as art students for Eichi, and stay in the village. The two find out from Eichi that the boy from the dam was neither Monster nor human, and people call him a demon, and despise him. The next day, Sayako found a girl with a large jewel around her neck, and decided to try and steal it. Meanwhile, another robot creature appears in the valley, of which Bocca quickly drives away. While he tries to chase it, he encounters a whole bunch of mice, who attack him. With an odd sound, he realized he was saved by the girl Sayako met. When she saw he was a Melos Warrior, she quickly drew her bow, and pointed an arrow at him. Exactly what's up with this place?

Bocca learns that the girl who had aimed her arrow at him is named Tone. And she appears to hate Melos Warriors. He also learns that the technique that she uses to drive the "working mouse away" is a Melos technique. However he notices that she has no stigmatic mark on her arm. On learning of Tone's return to the valley, Miri Kanaya becomes agitated and recalls back her earlier years, of how and why she joined the Monster Union. The relationships between her, Tone, Eichi Hikoyama and the blue-eyed boy are also revealed.

Eichi completes the mural on the dam's wall, he tells Tone that the rest will be left to them before he leaves. Miri under the orders of the Monster, rides her 'Rat Monster' for a final battle with Bocca. Bocca uses the "Crying Bow String" technique to defeat Miri. The Dam is destroyed to reveal Eichi's final work. When Tone and Bocca part their ways, it is revealed that the blue eyed boy is Tone's Aibar machine. Bocca also spots Tone's stigmatic mark on her thigh.