The children of the Momoyama family are outstanding. There is Natsumi, the elegant school nurse; Haruki, the renowned author; Arima, the athletic and teasing older twin; and Chihiro, the average-looking younger twin.

When they were just 10 years old, Arima and Chihiro heard their parents say, "We will tell them the truth when they are 16." Since then, they have lived each day all too aware that one of them was adopted—and, with her mousy hair and lackluster looks, Chihiro suspects that she is the adopted child.

But Arima never fails to cheer Chihiro up from this; he promises to stay by her side regardless if they were blood-related or not. However, when the fateful day comes, the truth they learn sends their world into upheaval, forcing the "twins" to rethink the bonds they made until now.