Months have passed since the fierce battles of PRISM.1, where Shin Ichijo delivered a shocking performance despite having no memory of it. After finally giving his fans the show he truly wanted to perform at the Rose Party, Shin remains burdened by a lingering sense of unease that he cannot shake. Hoping to lift his spirits, the other members of the Prism Star unit SePTENTRION invite him to board a time machine secretly developed by the Juouin Group. Their destination is the Dear Crown Christmas Live, the very first concert the seven of them performed together.

The group eagerly tours past Prism Shows, including legendary performances by the three greats known as the Big Three: Hijiri Himuro, Jin Norizuki, and Rei Kurokawa. However, their joyful journey takes an unexpected turn when the Prism radiance goes out of control. A rift in the space-time continuum opens, and the seven members of SePTENTRION fall into a mysterious and dangerous place called Mascot Hell. There, they encounter numerous other Pretty Boys who have wandered in from different worlds across the Pretty Series franchise, including characters from Pretty Rhythm, PriPara, Kiratto Pri☆Chan, and Waccha PriMagi!.

As these worlds, which should never have crossed paths, become connected by the power of Prism, a final judgment known as the Hell's Gods' Star descends upon them. Meanwhile, a grave threat emerges. To erase the soul of the Prism Messenger Shine sleeping within Shins body, as well as to eliminate Rui Kisaragi who abandoned his mission to protect Shin, the Prism World sends its final envoy, a mysterious figure named Ai Kannazuki. Shin and his friends must fight to protect Shine, uncover the true meaning of the Prism Act, and find a way to return home, all while unraveling the mystery behind a Prism Show that has been erased from history.

The main characters include the seven members of SePTENTRION: the determined Shin Ichijo, the kabuki-trained Yukinojo Tachibana, the street-style performer Taiga Kougami, the heir Kakeru Juuouin, the costume designer Leo Saionji, the family-oriented cook Minato Takahashi, and the aspiring songwriter Yu Suzuno. They are joined by a vast ensemble of male idols from across the Pretty Series timeline, as the film celebrates the franchise's 15th anniversary. The narrative is structured into several branching routes that change on a weekly basis during theatrical release, including Our Youth Over The Rainbow, The Legendary King Cup, The Miraculous Queen Cup, and The Story of Beginnings: Dreams and the Future, each focusing on different groups of characters from the franchises history.