With her mother’s remarriage, Matsuri Amamiyabecomes the younger stepsister of Haruka, Azusa, and Ritsuto, triplets who make up the wildly popular group, Terzetto. She had always dreamed of having brothers and being close to them, but is disappointed to learn that the siblings don’t get along at all.
The truth is, Azusa the second son wishes he and his brothers were closer. So Matsuri decides to help him make his wish come true. In appreciation for Matsuri’s efforts to bring the two families together, a welcome party is planned. The triplets join in the preparations, but as usual, end up fighting. When Matsuri suddenly collapses, the worried triplets call a truce.
A family that gets along is more fun than one that argues, so Matsuri is determined to intervene each and every time. While not ideal, things seem to be progressing well, when one day, she receives a confession of love from Azusa...!