Saikin, Imouto no Yousu ga Chotto Okashiinda ga.

Synopsis

The episode begins with Mitsuki asking Hiyori who she is and asking why she can't cross over. She responds by taking Mitsuki through time and space to steps leading to a big golden gate. Once there she explains that in life she loved Yūya but died before she could confess her feelings. She tells Mitsuki that in order for her to cross over she wants to do "lovey dovey" things with him. Each time this happens (from Hiyori possessing her or Mitsuki's own actions) the heart on the chastity belt fills up a little. Each time the heart is filled a step is added that leads closer to the golden gate, once full the gate will open allowing Hiyori to cross over. After waking up and thinking it was all a dream, she is then disappointed to find otherwise and then heads off to school. While at school Mitsuki lets Hiyori possess her for gym class and as a result she does really well. When Hiyori tells her that she is doing this to try and draw attention from Yūya Mitsuki protests. Hiyori continues to say that if after a while Mitsuki does nothing the heart on her belt will drain completely causing both of them to die and go to hell. While at school Mitsuki has the sudden urge to use the restroom. Wrongly thinking that she is sick Yūya takes her to the nurse which ultimately causes Mitsuki to wet herself in front of him. Crying, and embarrassed Mitsuki doesn't know how she can face him but is cheered up by Hiyori. At the same time Yūya has an accident at the grocery store which causes him to take a bath at home with the door's lock broken. After his bath Mitsuki accidentally sees him naked and then goes to take her own bath. When she tries to get out she discovers that the door is broken, Yūya hears this and explains the door can only be opened from the outside. Yūya says to her that he will open it a crack so she will able to get out. Hiyori takes the opportunity and possess Mitsuki pulling him in there with her which shuts the door and traps them both inside the bathroom.

(Source: Wikipedia)