Three months after Poppin’Party's self-sponsored live show, the local live house owners, including CiRCLE's Marina Tsukishima, organize the BanG Dream! Girls Band Challenge, a tournament with the final taking place at the Nippon Budokan. Poppin'Party and Afterglow express their interest in participating, and although Kasumi invites Rokka to join them, she declines as she is still searching for a band. Meanwhile, CHU2 struggles to find a new guitarist for RAISE A SUILEN until she watches Rokka's performance at the cultural festival. CHU2 invites Poppin'Party, Roselia, and Rokka to RAS' latest concert, where she proclaims the band's entry into the Girls Band Challenge and openly offers Rokka an audition, much to her dismay.

After not performing in RAS' encore, Rokka is chased by CHU2 and PAREO in school and around town the next day, wanting to scout her for RAS. In the meantime, CHU2 meets with Yukina, reiterating her challenge to Roselia. Seeing similarities in the bands' philosophies, Yukina decides to have Roselia enter in the BanG Dream! Girls Band Challenge competition. Later in the night, after seeing Rokka practice her guitar in the Galaxy live house, Masking takes Rokka to CHU2's studio, where she performs alongside RAS. Unimpressed with Rokka's lackluster performance, CHU2 decides not to include her in RAS, explaining she cannot accept those withholding their skills to satisfy her.

The preliminaries for the BanG Dream! Girls Band Challenge begin with the top two teams in vote count going to the finals at the Budokan. Roselia and RAS begin performing their live shows while a zoned-out Rokka comes to terms with her failed RAS audition. Meanwhile, Poppin'Party write a new song for their first live show in the preliminaries which they perform in front of a live audience and Rokka. Motivated by Poppin'Party's new song, Rokka quickly rushes to RAS' studio where she is given one last chance to audition. Impressed by Rokka's passionate performance, CHU2 adds her as a (temporary) guitarist, ordering Rokka to become a guitarist worthy of RAS.

With Rokka joining as the (temporary) guitarist of RAISE A SUILEN, the band heads to a film studio to shoot a music video (MV) of their new song. Afterwards, they return to the studio to play their new song, but Rokka struggles to play together with the other band members. After a chat with Masking in a ramen shop, Rokka keeps playing until the song is perfect. The next day, the band is impressed by the PV they put together. During lunchtime, CHU2 inducts Rokka as a permanent member of RAS with the moniker LOCK.

Inspired by RAISE A SUILEN's music video (MV), Poppin'Party decide to make a MV of their own (dubbed PoPi-V). Taking cues from Pastel＊Palettes' MV and Rokka's ideas, Poppin'Party take random photos and videos with Rimi and Arisa putting the video together. After many sleepless days and nights, Poppin'Party are amazed by the MV they produced. They nervously upload the MV on the competition website but are excited by the likes they receive straight away. Later in the night, CHU2 confronts Roselia and proposes a band battle with RAS, which Yukina accepts.

RAS and Roselia prepare for the band battle that is held at Dub Live House. Despite Roselia preparing for the "Over the Future" concert, a precursor event to FUTURE WORLD FES, Yukina still wants to perform with RAS as soon as possible. Sayo leaves, believing that Roselia are overloading themselves practicing for two events at once. However, after talking to CHU2 and finding out why she wanted to set up RAS, Sayo comes back, intent on beating them. On the night of the band battle, RAS and Roselia perform. Announcing the verdict after the performances, RAS wins the band battle with CHU2 exclaiming that RAS is better than Roselia.

Reeling after their defeat to RAS in the band battle, Roselia reflect upon their performance and prepare for the "Over the Future" concert. Sayo practices intensely for the concert but collapses during a break from practice due to overwork and fever. Sayo recuperates in her bedroom with Hina and the other Roselia members visiting and preparing a meal for her. Yukina chats with Sayo about why she accepted CHU2's band battle challenge: to learn something from other bands such as RAS in order to grow as a band. After Sayo recovers and doing one last practice session, Roselia performs in the "Over the Future" concert. Yukina thanks the fans for their ongoing support before starting their performance.

All the old shopkeepers from the Jizo-dori shopping district break their backs just before their trip to the hot springs, so everyone (except PAREO and CHU2) go in their place. After settling into their rooms, the characters from different bands interact and socialize with each other. In particular, Rei and Kasumi meet, where they praise each other's performances as RAS and Poppin'Party respectively. Some of the characters also rest in the hot springs with some staying on for too long, resulting in them overheating. In the end, Afterglow inform Kasumi and the others that Poppin'Party's music video gained over 10,000 views which they celebrate before tending to the overheated characters.

Hoping to gain more votes, Poppin'Party plan a three-week schedule of live performances. In addition to Galaxy, they play at CiRCLE and Dub alongside Roselia and RAS respectively. Despite having fewer votes than both bands, they enjoy their newly-gained momentum as they reach sixth in the polls. Although RAS offer their support to Poppin'Party, including baking a cake for Tae's birthday, they fall behind Roselia in the voting. In response, CHU2 calls an impromptu meeting in which she announces that RAS will play in other venues to force other bands to withdraw. She also takes control of everyone's calendars and instructs the band members to follow her orders and to not socialize with other bands. After an argument where CHU2 questions everyone's commitment to RAS, an incensed Masking walks away. PAREO attempts to console CHU2 but is harshly criticized as being useless.

After Masking leaves the impromptu RAS meeting, Rokka looks for her but to no avail. The next day, after receiving advice from Kasumi, Rokka searches for Masking again, finally finding her in her school. Meeting in front of the Budokan, Rokka praises Masking for helping her and tells her to meet with CHU2 to make up which Masking initially becomes hesitant before agreeing. Meanwhile, Rei meets Tae where she talks to her whether she still cares about RAS; Tae tells her that as the vocalist of RAS, she has the power to change what RAS will do in the future. RAS (sans PAREO) reunite as a band to perform, but feel anxious and distracted due to PAREO's absence and do not perform well. While Rokka and Masking head to Chiba to find PAREO, Rei talks to CHU2 about needing to work together in the best interests of the band. After finding out that CHU2 wrote off PAREO for being useless, Rei and CHU2 also head out together to find her.

In Chiba, Masking and Rokka search for PAREO by visiting her school. When they show her classmates a photo of her, they learn of PAREO's true identity as Reona Nyūbara. The two spot Reona as she is walking home, prompting her to run away and them to give chase. Upon catching up to her, they learn of her background as an academically successful but meek girl who regularly posts keyboard covers of Pastel＊Palettes songs online. Despite Rokka and Masking's efforts, a disillusioned Reona proclaims PAREO no longer exists until she spots CHU2 and Rei running up to them. After CHU2 asks for forgiveness and is accepted, the band performs a song for her birthday back in the studio. Eventually, the Girls Band Challenge voting closes with Roselia leading, while Poppin'Party and RAS are tied for second.

Bringing Poppin'Party and RAS together at Galaxy, Marina announces that both bands will proceed to the finals at the Budokan, to everyone's relief and excitement. Each band then prepares to perform in the Budokan which includes a dress rehearsal at CiRCLE. Along the way, Poppin'Party prepares a new song that they want to play with Roselia and RAS, both of whom accept. Meanwhile, Asuka volunteers to help in the Girls Band Challenge, including creating a poster for the finals and interviewing the bands for her blog. The night before the finals, Kasumi sees Asuka proofreading her blog post. Asked to explain how she finds sparkling, heart-pounding moments in her life, Kasumi recounts how she found these moments by forming Poppin'Party and meeting and making new friends over the past year.

The Girls Band Challenge finals arrive as Poppin'Party, RAS, and Roselia assemble at the Budokan, where they help set up the stage. Before Poppin'Party's performance, Rimi confides to her band that she is hesitant to play as the finals symbolize the end of their journey up to that point. However, she and the band are uplifted when they hear the crowd recite their pre-performance chant. Poppin'Party and Roselia perform new songs, while RAS play a rendition of Chiyu's birthday song. The audience votes for the winner, with Roselia claiming the Grand Prize; to the others' surprise, RAS receives the Best Performance title while Poppin'Party is named Best Band. The three bands close out the event by playing their collaboration song.