Hikikomori are persons who have become so socially isolated that they refuse to leave their houses for weeks or even months at a time. It's more than a word for Sasami Tsukuyomi, who is struggling to complete her first year of high school while being a shut in. Fortunately, she lives with her elder brother Kamiomi, who happens to be a teacher at the school where Sasami is scheduled to be. Not to mention her "Brother Surveillance Tool," which allows her to watch the outside world through her computer and, presumably, allow her to re-acquaint herself with interacting with others. What it mostly does is allow her to see her brother's interactions with the three extremely strange Yagami sisters, who appear to be mysteriously linked.