The protagonist, "Souta," and the neighboring sisters, "Hoshika" and "Rikuka," are childhood friends who have been close like siblings since they were young. However, as they grew up, he began to develop romantic feelings for the sisters. But because they had been like siblings since childhood, he was agonizing over whether his feelings were LIKE or LOVE.

One day, as summer vacation was approaching, he received a love letter. This letter became the catalyst for a change in his relationship with his "sisterly" older sisters.

A sweet and erotic lesson begins, where he's thoroughly spoiled by his serious older sisters?!