The protagonist, Souta, and the sisters next door, Seika and Rikka, are close childhood friends. However, as he grows up, he begins to feel attracted to the sisters. Since they've been like brother and sister since they were little, he doesn't know whether his feelings for them are "like" or "love", which makes him anxious. One day, just as summer vacation is about to begin, he receives a love letter. The letter triggers a change in his relationship with the two sisters. This is the start of naughty lessons, being pampered by the two sisters who are getting serious about him!