Kanzaki Mitsuki has been troubling herself over her relationship with her new family and her new older brother now that her mother's remarried; to top it off, her mother is following her new step-father to his post abroad.

Thanks to the ghost girl Kotobuki Hiyori, Mitsuki is forced to wear an indecently shaped chastity belt around her crotch; in order to get the device off, Mitsuki will have to satisfy Hiyori.

Mitsuki and Yuuya are both locked in the bathroom naked; the one who saved them was Kiritani Yukina, aka Yuki-nee, Yuuya's childhood friend who moved back in next door.

After declaring that Mitsuki is a failure as a little sister, she decides to set an example through action; she borrows Mitsuki's body, fixes breakfast in an apron and goes shopping together with her brother.

By coincidence, Mitsuki meets up with Yuuya, Yukina and the others at a water park; due to her worries about the chastity belt she covered with her pareo, she doesn't go in the water.

Yuuya is left confused by Mitsuki's unstable attitude; he doesn't really understand the girl with wings that he keeps seeing from time to time either; thanks to her chastity belt, Mitsuki is depressed from the embarassment of getting caught.