"Love Live! Sunshine!! The School Idol Movie Over the Rainbow" serves as the concluding chapter to the "Love Live! Sunshine!!" anime series, continuing the journey of Aqours, the school idol group from Uranohoshi Girls' High School. The film picks up after the events of the second season, where Aqours successfully saved their school from closure by winning the Love Live! competition. Despite their victory, the group faces a new challenge: the impending graduation of the third-year members—Chika Takami, Riko Sakurauchi, and Kanan Matsuura. This looming separation creates a sense of uncertainty about the group's future.

The narrative begins with Aqours traveling to Italy for a school trip, which doubles as a celebratory reward for their Love Live! triumph. The trip serves as a backdrop for the characters to reflect on their journey, bond further, and confront their anxieties about what lies ahead. The third-year members, in particular, grapple with their impending graduation and the emotional weight of leaving behind their underclassmen. Meanwhile, the first- and second-year members struggle with the idea of continuing Aqours without their seniors, questioning whether they can uphold the group's legacy.

Throughout the film, the characters engage in heartfelt conversations and introspective moments, exploring themes of growth, perseverance, and the bittersweet nature of transitions. The trip to Italy also introduces new experiences and challenges, including a performance at a local festival, which becomes a pivotal moment for the group. This performance symbolizes their unity and determination to move forward, even in the face of change.

As the story progresses, Aqours decides to hold one final concert as a complete group, aiming to create a lasting memory before the third-years graduate. The concert serves as both a celebration of their achievements and a farewell to the current iteration of the group. The film concludes with a sense of hope and renewal, as the underclassmen resolve to carry on Aqours' legacy, inspired by the bonds they have formed and the lessons they have learned.

The main characters—Chika, Riko, Kanan, Dia Kurosawa, You Watanabe, Yoshiko Tsushima, Hanamaru Kunikida, Mari Ohara, and Ruby Kurosawa—each contribute to the emotional core of the story, with their individual struggles and growth reflecting the broader themes of the narrative. The film balances moments of lighthearted camaraderie with poignant reflections, providing a fitting conclusion to Aqours' story while leaving room for the next generation to shine.