From here:
Just before entering university, Yanagiba Masahiko's father died in a car accident. His mother died when he was two, which makes him now really an orphan. But even before the death of his father he didn't have a family life, his father being busy or abroad.

When his mother's younger brother and his wife decide to invite him to live with their family, his life takes a turn. But it is not going to be the normal family life he is imagining himself. What is the "terrible" secret that lead his parents to break the bonds with his aunt and uncle? What could be the true nature of his cousin, Shion? And last but not least, will this "terrible" secret eventually get Yanagiba too?