While taking out the trash one night, Kazuki Itou sees a dirty girl in a gothic lolita dress. As he passes by, she asks, "Would you have a little sister?" and Kazuki affirms that he does. After a few more encounters with the strange girl, he realizes that she has gained access to his apartment and has begun stalking him, intent on actually becoming his little sister.

So what does this mean for Kazuki's real sister, Hikari? If the local urban legend about the demon lolita is true, she will kill Hikari and take her place. But it may turn out that Hikari's supposed fate is preferable to what the lolita has in store for Kazuki...