"Project A-ko 3: Cinderella Rhapsody" is the third installment in the "Project A-ko" series, continuing the comedic and action-packed adventures of its titular character, A-ko Magami, and her friends. The story revolves around a school festival at Graviton High, where A-ko, B-ko Daitokuji, and C-ko Kotobuki are tasked with organizing a play based on the fairy tale "Cinderella." A-ko takes on the role of the prince, B-ko plays Cinderella, and C-ko is cast as the fairy godmother. However, the production quickly spirals into chaos due to the characters' conflicting personalities and B-ko's obsession with outshining A-ko.

As the festival approaches, B-ko becomes increasingly determined to make the play a success, even if it means resorting to underhanded tactics. She manipulates the script and uses her family's wealth and influence to ensure her performance is flawless. Meanwhile, A-ko, who is more interested in physical activities than acting, struggles to keep up with B-ko's demands. C-ko, ever the carefree and clumsy friend, adds to the chaos with her unpredictable behavior.

The narrative takes a fantastical turn when a mysterious alien artifact, resembling a glass slipper, is discovered. This artifact grants its wearer immense power, leading to a series of misunderstandings and conflicts. B-ko, driven by her desire to surpass A-ko, becomes obsessed with obtaining the artifact, while A-ko and C-ko inadvertently get caught up in the ensuing chaos. The situation escalates when the artifact's true nature is revealed, and the characters must work together to prevent a disaster.

The film blends slapstick comedy, over-the-top action, and science fiction elements, maintaining the series' signature tone. A-ko's superhuman strength and B-ko's technological prowess are showcased in exaggerated battles, while C-ko's innocence and clumsiness provide comic relief. The dynamic between the three main characters remains central to the story, with their rivalry and friendship driving much of the plot. The school festival setting serves as a backdrop for the characters' antics, creating a lively and chaotic atmosphere.

"Project A-ko 3: Cinderella Rhapsody" builds on the established relationships and conflicts from the previous films, offering a self-contained story that continues to explore the themes of rivalry, friendship, and the absurdity of teenage life. The film's narrative structure allows for both character-driven moments and action-packed sequences, ensuring a balance between humor and excitement. The inclusion of the alien artifact adds a layer of science fiction to the otherwise grounded setting, further expanding the series' eclectic mix of genres.