Kasumi is shocked to discover that her older sister's fiance is the man she first fell in love with five years ago in high school. At the time, he was her high school teacher and ultimately quit his job racked with guilt over their relationship. Kasumi was deeply hurt and struggled for a long time to overcome her sadness. Now the two of them must come to terms with their secret past before the ones they care about in the present are hurt.

　　高中女生夏澄，爱慕单身英俊的老师藤堂直，同校清一却对夏澄一往情深。而当藤堂克制不住自己的感情，与夏澄淡淡的一吻之后，却忽然不知道去往。五年后，夏澄的姐姐朋子将男朋友回家时，夏澄赫然发现，姊姊的未婚夫竟是朝思暮想的初恋情人藤堂。故事就围绕绕绕爬四个人所著的爱开始展开…。

渡部篤郎×深田恭子×和久井映見共演。元教師と教え子、そして姉の婚約者と妹…その“禁断の恋”を描く大人のラブストーリー。高校教師の直（渡部篤郎）は、自分に無垢で一途な想いを抱く女子生徒・夏澄（深田恭子）に惹かれ、一度キスだけを交わす。だが直はその学校を辞め、夏澄の前から姿を消してしまう…。そして５年後、直との思い出を胸に秘めたまま生活していた夏澄に、姉・朋子（和久井映見）が家族に婚約者を紹介したいと宣言。そこに現れたのは直で…。