Two years ago, the death of Naoto Watari's parents left him as the sole caretaker for his younger sister, Suzushiro, and forced him to move in with their aunt. So devoted is he to Suzushiro that he refuses to join clubs or even hang out with anyone, earning him snide comments and the title of "siscon" at his new school. But as long as Suzushiro is safe and happy, Naoto does not care what everyone else says.

However, his sneering classmates may turn out to be the least of his worries. The sudden transfer of a girl named Satsuki Tachibana brings back traumatic memories from his past—a past she had a hand in making so unpleasant. Nasty secrets, unknown to Naoto, begin to surface with her arrival, threatening his new life.