Under Tokyo district Machida, there was a mysterious movement of soil: the land subsided, creating a huge cave. In addition, this event coincided with the appearance of a monstrous serial killer, who killed almost all the population of an apartment building. Rammaru and Saya Shindo miraculously survived this double disaster. It's been five years. The brother and the sister decided to go in the footsteps of their father, a police officer. And if it is just good honest work for Saya,