3年前の8月31日、巨大な宇宙船「母艦」が突如として東京に襲来し、世界は終わりを迎えるかに見えた。その後、絶望的な状況は次第に日常に溶け込み、上空に母艦が浮遊する異様な光景が当たり前となっていた。そんな中、女子高生の小山門出と「おんたん」こと中川凰蘭は、担任教師の渡良瀬や仲の良い友人たちとともに何気ない学生生活を送っていたが……。
Three years ago, on August 31st, a gigantic spaceship called the "Mother Ship" 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 their homeroom teacher Watarase and their close friends...
