What Is Going On With My Sister?

Mitsuki and Yuya become stepsiblings because of the remarriage of their parents. Mitsuki finds it difficult to fit in the new family while Yuya does not know how to approach the new step-sister. One day, Mitsuki is attacked by a spirit named Hiyori who is fond of Yuya. Hiyori uses the body of Mitsuki and makes a plan to go to heaven.