An immoral story of flirty-dirty between a student and his younger sister.
A service-minded sister Mikan is always steady taking care of her oniichan.
But in heart, she wants his affection since her age is as such.
When they take bath together for the first time in a while, they start getting
into an immoral relationship. But he happens to see that she plays with her male classmate.
Does she have a boyfriend? What will happen to their immoral love...?