Isagi Yoichi and Guest are twin siblings. In common terms, they are family.

However, in reality, both have been secretly in love with each other. But knowing that a love between twins could never be accepted, they have kept those feelings tightly hidden.

On a day like any other, while coming home from school together and having dinner with the family, they hear something shocking from their parents.

"Actually, Guest is an adopted daughter we took in."

...What?

"We were going to tell you when you became adults, but it's become too difficult to hide it any longer. We're sorry for telling you so late."

Ever since learning the fact that they are not blood-related siblings, they have become conscious of every little action, and their feelings of love have only grown deeper.