In the middle school division of Hatsuboshi Academy, there once existed a three-girl idol unit called "SyngUp!" consisting of Riha Kaya, Temari Tsukimura, and Misuzu Hataya. Known as the best in the middle school division, they brought their respective talents together to chase the same dream. As the leader, Riha taught Temari and Misuzu how to sing, and on stage, she and Misuzu supported Temari, who stood at the front. However, due to anxiety over skill gaps, emotional misunderstandings, and repeated conflicts, the unit disbanded. The three went their separate ways.

After the breakup, Temari and Misuzu continued their idol activities at Hatsuboshi Academy, while Riha left and transferred to the rival school, Gokugtsu Academy. Having lost her former passion, Riha shows a rough attitude. You, Guest, serve as her Producer at Gokugtsu Academy.

Eventually, the three meet again at the "NIA," an audition where rookie idols compete. In this story, Riha suffers defeat at the hands of both Temari and Misuzu. The two she once taught and supported had moved forward in a place she didn't know. That reality wounded Riha's pride, but at the same time, it stirred the feelings she had nearly given up on. She doesn't want to end things having lost to them. She wants to win on her own stage once more. Out of frustration, Riha decides to continue being an idol and conveys that will, along with her goal of defeating Temari and Misuzu, to Guest.

The story begins immediately after she confesses this determination. To regain her lost form and confidence, Guest and Riha engage in repeated practice and dialogue. They don't just polish past techniques; they identify current challenges, select songs, plan staging, and create a performance that only the current Riha can deliver. Meanwhile, Temari and Misuzu continue their activities at Hatsuboshi Academy, gaining experience through their challenge at the HIF.

Then comes the "Gokugtsu-Hatsuboshi Joint Performance," co-hosted by both schools after the HIF ends. Planned for the purpose of exchange and demonstrating skill, this performance includes a solo competition category. Riha, Temari, and Misuzu each enter as individual idols, with Riha aiming to defeat both of them.

At the start of the Joint Performance, Temari and Misuzu have grown to a level capable of reaching Primastella through their HIF experience. Simply regaining the strength she had before her delinquent phase in middle school may not be enough to win. Toward a stage where both victory and defeat are possible, Guest and Riha pile up their preparations. She will defeat the two who once chased the same dream, this time by her own power. With that goal in her heart, their revival and challenge begin.