The cube's name is "Kado".
A strange being called Yaha-kui zaShunina appears from within Kado and tries to make contact with humanity. Shindou, who was been absorbed by Kado, ends up taking on the role of mediator between Yaha-kui zaShunina and humanity. The Japanese government sends its own negotiator, Saraka Tsukai.
Who is Yaha-kui zaShunina? What does he want?