When QUARTET★NIGHT is chosen to perform as the Triple S Opening Artist, they decline the opportunity after ST☆RISH and HE★VENS give equally compelling performances. Raging Otori and Shining Saotome propose another musical showdown to determine the opening artist between the three groups in which all three of them must sing songs written by Haruka! Who will win the decisive concert and the honor of performing at the Triple S?

The members of ST☆RISH and HE★VENS are shocked to learn they will be singing duets with members from the other group to prepare for the decisive concert. Meanwhile, QUARTET★NIGHT appears to be experiencing some discord of its own.

As the first to do his duet, Tokiya realizes Eiji's talent. Worried that ST☆RISH's talent isn't enough, he crafts a plan for the team to take their competition more seriously.

The second group of the Duet Project is Syo of ST☆RISH and Yamato of HE★VENS. Syo is excited to sing a duet with Hyuga Ryuya's younger brother, but a powerful rivalry with his older brother surges through Yamato's heart.

The third duo of the Duet Project are Cecil of ST☆RISH and Shion of HE★VENS. Trying to follow up on Tokiya and Syo's success, Cecil aggressively approaches the project, but Shion closes off his heart more and more as though in response to Cecil's passion.

The fourth duo of the Duet Project are Ren of ST☆RISH and Van of HE★VENS who are also slated to play rivals in a TV drama. When Van begins aggressively pursuing Haruka, Ren heads straight into battle with his own passionate feelings hidden in his heart.

Nagi persists to perform their duet project with musical instruments: Natsuki playing the violin while Nagi plays the viola. This causes Natsuki to start recollecting unpleasant memories prompting "Satsuki" to step in.

Masato is called back to the Hijirikawa family after his father collapses which puts a strain on his duet project with Kira. Upon returning home, Masato is pressurised into taking over the family business prompting Ren and Kira to confront him about his true ambitions.

The theme of Otoya and Eiichi's song is a "soul-shaking song." When Eiichi explains to Otoya that he must face his true self that exists deep inside his heart to do so, he stoically engages in writing the lyrics. However, gradually he becomes less and less like himself.

Leaving behind a completed duet, Otoya disappears. Having been exposed to a side of Otoya they'd never known before through his voice, as the members of ST☆RISH search for clues that will lead them to Otoya, they arrive at the truth behind his personal history.

After reuniting in the sunflower field, ST☆RISH successfully makes it to their live broadcast while becoming the talk of the town. With the Duet Project finished, ST☆RISH and HE★VENS are set to start working towards the Decisive Concert. However, a plot to drive them towards despair is approaching.

The emotions and hopes of ST☆RISH, QUARTET★NIGHT, and HE★VENS are illuminated by a morning sun rising in a clear sky. As Haruka prays for similar things, the Decisive Concert is about to begin.

ST☆RISH is up next and the tension is thick. Who will come out victorious in this action-packed showdown?