Shindou Yukijiro is a skilled negotiator working for the Ministry of Foreign Affairs. At Haneda Airport, when the passenger plane that Shindou was on was about to take off, a mysterious huge cube appeared in the sky. "That thing" rapidly grew in size and swallowed up the passenger plane with 252 people on board. The name of the huge cube is "Kado". The mysterious existence Ahakui Zashunina emerged from Kado and tried to contact humans. Shindou, who was absorbed by Kado, accepted the task of mediating between Ahakui Zashunina and humans. On the other hand, the Japanese government also sent international negotiator Yosaraka to the scene as a representative. Who is Ahakui Zashunina? And what is his goal?