Seikai Suru Kado

Shindou Koujirou is a highly-skilled negotiator working for the Ministry of Foreign Affairs. As his plane at Haneda airport prepares to take off, a huge mysterious cube appears from the sky. "It" expands rapidly, and absorbs the passenger plane and its 252 passengers.
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, Tsukai Saraka. Who is Yaha-kui Zashunina? What does he want?