After Uranohoshi Girls' High School shuts down, the third-year students—Dia, Kanan, and Mari—graduate, leaving Aqours with only underclassmen. In search of a practice spot, the group visits their upcoming school, only to find it deserted. It turns out they've been relocated to a branch school due to concerns about their impact on the clubs. Determined to prove themselves, Aqours aims to show their dedication. Meanwhile, the third-years vanish during their graduation trip, prompting the rest to embark on a journey to find them and change their new school's perspective. Love Live! Sunshine!! The School Idol Movie: Over the Rainbow follows Aqours' quest to locate their missing friends and sway the school's opinion.