With her mother’s remarriage, Amamiya Matsuri becomes the younger stepsister of Haruka, Azusa, and Rihito, 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.