Three years ago, On August 31st, A gigantic spaceship called the Mothership suddenly attacked Tokyo, and it seemed like the world was coming to an end. After that, the hopeless situation gradually became part of everyday life, and the strange sight of a mothership floating in the sky became commonplace. Meanwhile, high school girls Kadode Koyama and Oran Nakagawa, also known as "Ontan," are living a casual student life with her homeroom teacher, Watarase, and her close friends.