Sachi and Megumi are surprised that Yuna can see them, as nobody else has ever been able to. This leads the ghosts to ask Yuna for help, with a twist...the two ghost girls are a couple, and they can't move on to the afterlife until they figure out how to "do it" so to speak. Thus, they want Yuna to go around the school and play matchmaker to several potential couples, so that they can "observe" and learn how to consummate their relationship.
