When I return to my hometown after a long time, my childhood friend and sister, Ai, greets me energetically. That night, as I get into bed, Ai asks, "Onii-chan... can I... sleep with you...?" As I am confused, she confesses, "...I like Onii-chan. But I know that's wrong. But... I can't suppress these feelings." Although I am bewildered by her growth from a child to a young girl, my brother, who feels the same way, cannot suppress his feelings and connects with his sister...