Kurihara Satoko was satisfied with daily life with her husband and son. In fact, the son wasn't their own child, but it wasn't matter for them. After they had tried some treatment for sterility for a long term, they once gave up to have a child. However, they came to know about "adoption", and they took Asato as their son. She couldn't imagine the life without Asato, and believed her happiness continued forever.

One day, Satoko got an unexpected call from Katakura Hikari, who insisted that she was Asato's biological mother. Satoko was getting lost into the past because of this call...