Mitsuki and Yuya become stepsister and stepbrother because of their parents' marriage. Mitsuki can't get along with her new family and Yuya doesn't know how to approach his new stepsister. One day, Mitsuki becomes possessed by the spirit of Hiyori. Hiyori likes Yuya and believes Yuya can help her get to heaven. Hiyori places a leather chastity belt upon on Mitsuki and a plan takes place to help send Hiyori to heaven.
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, leaving her all along with her new step-brother, Yūya, and only making her troubles worse...
Thanks to the ghost girl Kotobuki Hiyori, Mitsuki is forced to wear an indencetly shaped chastity belt around her crotch. In order to get the device off, Mitsuki will have to satisfy Hiyori so she can pass on by using her own body to experience all the kinds of things lovers would with Yūya.
Mitsuki and Yūya are both locked in the bathroom naked?! The one who saved them was Kiritani Yukina, aka "Yuki-nee", Yūya's childhood friend who moved back in next door. With the appearance of a powerful rival who's an older woman, a childhood friend, and has big breasts unlike Mitsuki, Hiyori panics.
After declaring that Mitsuki was 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. Mitsuki watches how well they get along in her spirit form, feeling mixed feelings about it.
By coincidence, Mitsuki meets up with Yuuya, Yukina, and the others at a water park. But due to her worries about the chastity belt she covered with her pareo, she couldn't go in the water, and she was left sulking by the poolside as she watched the others happily having fun.
Yuuya is left confused by Mitsuki's unstable attitude. He doesn't really understand the girl with wings (Hiyori) that he keeps seeing from time to time either. Meanwhile, thanks to her chastity belt, Mitsuki was extremely depressed from the embarassment of getting caught in it.
Ayaka invites Mitsuki to join her in observing the various sports clubs, and Mitsuki agrees. When she borrows Hiyori's ability, Mitsuki is able to display incredible physical prowess, but she never had any interest in such clubs.
Yuuya recieves tickets to the aquarium from Neko. The moment she sees them, Hiyori hijacks Mitsuki's body, and decides that the two of them are going to the aquarium together.
Tomorrow there's a math test, and that's when Aunt Nanami finished moving in to the Kanzaki household and set out the kotatsu. Mitsuki decided to study under the warm kotatsu, but soon begins falling asleep instead.
After fighting over a trivial matter, Hiyori throws a fit and leaves. Mitsuki even enjoys the chance to finally have some time alone to herself for the first time in a while. However, during her dinner with Yuuya, she notices her own heart aching at his little behaviors.
Mitsuki won tickets to a hot spring at a lottery, so she ends up traveling together with Yuuya as well as Nanami, Yukina, Ayaka, Neko, Torii, and his little sister. Once they arrive, the girls head straight for the outdoor bath, but Mitsuki wasn't feeling very happy.
Mitsuki's his little sister, and Yukina is an older sister figure. To Yuuya, both of them are precious "family members". But on one morning of their stay at the hot-springs, Neko hypnotizes Yuuya to "be more honest", and he calls Mitsuki "really cute" and gropes Yukina's breasts.