Having failed his college entrance exam, Ao Toruno ends up living by himself and attending cram school. It is then that he encounters a middle-aged woman, still clad in her office suit, drunkenly passed out at the bottom of his apartment's stairway. Despite not knowing her identity, Ao offers his apartment as a place to rest. Everything seems normal for the most part—but at the flick of a switch, things become intimate for the two, and Ao finds himself stuck in-between focusing on his studies and trying to put up with the advances of a nameless "onee-san."

[Written by MAL Rewrite]