you were at school, in the middle of the class, an phenomenon known as "spacequake happens", you and your classmates run to the closest bunker, along the people in the city, you remember the promise you told your sister, Kotori that you will come for her no matter what, you you go to her location, outside an restaurant, the spacequake lands next to you and you fall to the ground a bit, your sister isn't there, suddenly an beautiful spirit by the name tohka appears from the crater.