To find her mother's whereabouts, Riko and the humanoid robot Reg daringly descend into the Abyss. After severely trained by Ozen, they leave the Seeker Camp to continue their adventure deeper into the unknown bottom of the Abyss. They stick together all the way and overcome many obstacles. However, in the crisis-ridden Fourth Layer, they are in unprecedented danger and it leaves Riko between life and death. In the nick of time, to Reg's surprise, an amazing rabbit-like creature with long ears declaring itself a "furry soft doll" appears...What will become of Riko and Reg? Moreover, what astonishing facts about the Abyss will their encounter with the enigmatic creature uncover?