Haruka meets the cast again after a period of not seeing each other. Shining Saotome announces that QUARTET★NIGHT will be the main act to perform in the sports event Triple S, and that Haruka will compose their song. ST☆RISH complains about this decision and decides that they'll have to prove themselves worthy by participating into a whole new project called Cross Units.
Each senior and underclassmen unit gets a variety show skit: Otoya, Tokiya and Reiji work in a daycare; Ranmaru, Masato and Ren take a stroll through town; Natsuki, Sho and Ai are confronted with a test of courage and Camus and Cecil drink some tea and try pastries at a tea shop. All the seniors show their skills leaving the juniors in admiration and thinking about how they can improve themselves. Later on, the whole cast goes to the beach.
Natsuki and Otoya are made into a Cross Unit and are cast as the main characters in a rock opera. Natsuki is put to play the part of a hot-headed manly guy while Otoya has to learn to be collected and cool. Both consult with their respective members (Syo and Tokiya) hoping to get some advice from them but without much luck. It is then that they both realize that they have been looking at things the wrong way and shouldn't be so worried about completely changing themselves. With the help of their friends and Haruka, they are able perform a brand new song in the play.
Ai becomes the main character in a new film. He works along with Syo and Natsuki both left in awe at how perfectly he does everything, however, Ai begins to feel something has changed within him. Ai ends up overworking himself and becomes ill but refuses to rest resulting in him collapsing. Both Syo and Natsuki, along with Haruka, put Ai to rest and in the process learn that he is a robot. After explaining, the three help Ai to finish the film.
Syo, Ren and Cecil are made into a Cross Unit and are selected to be featured in a commercial for a mobile phone brand. Things head off to a rocky start with each member's conflicting ideas about what the theme for the commercial should be. They end up getting into a heated argument but realize that their fellow member's ideas were just as good as theirs. In the end, with help from Haruka and their friends, they are able to come up with the perfect theme for the commercial and Haruka's new song.
Camus is chosen as the main act in the inauguration of the Shining Tower. Haruka, hoping to learn more about Camus to write him a new song, ends up working as his assistant. Although the tasks are difficult, Haruka does not give up determined to see a side to Camus in which he can do something earnestly seeing as he always does everything perfectly but without being earnest. Haruka's feelings are able to reach Camus and he performs his new song at the inauguration.
Ranmaru has decided to leave rock music as a way to move from his past and focus on his new life as a QUARTET★NIGHT member. Everyone opposes saying that rock is his life but he refuses to listen. Due to a past experience, Ranmaru is afraid of making the same mistakes again with his new found band members but Haruka teaches him that his love for rock and his band can be balanced without giving up on either.
Tokiya and Masato are set in a Cross Unit and work with each other in order to write lyrics for Haruka's song. Everyone thinks they are a perfect pair and look forward to their new song, but with them being the last to perform and everyone having high expectations, the pressure gets to them and they are unable to come up with any good lyrics. They try many things to be able to focus and relax but to no avail. Haruka then comes to them with some food revealing that everyone just wants them to do their best and that they are thinking about them. This gives Tokiya and Masato the encouragement they need to write the lyrics and perform their song.
Reiji takes a special liking to Haruka while Tokiya and Otoya feel overshadowed by his senior.
QUARTET★NIGHT and Haruka go on a trip to learn more about each other and write a song for Triple S. Haruka's kindness will inspire them to make a new song, they first they're able to make together.
ST☆RISH debates about what to do about Haruka since she's gotten close to QUARTET★NIGHT and they tell ST☆RISH they'll fight for keeping Haruka as their composer. As soon as she returns, they sing her a new song, "THANK YOU".
It's finally time for the Triple S concert, and QUARTET★NIGHT performs again (although in a longer and polished version) their song "Evolution Eve", leaving ST☆RISH, Haruka and the whole theater impressed.
QUARTET★NIGHT and ST☆RISH fight for the entry song of the SSS competition. As soon as the judges are deciding which one of the two contestants will win, HE★VENS has their first debut since their defeat to ST☆RISH during the UtaPri Award, debuting now as a 7-member band. The battle continues on for the confirmed 4th season.
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?