"Eiga Precure All Stars: Haru no Carnival" is a crossover film that brings together the magical heroines from multiple seasons of the "Pretty Cure" franchise. The story is set during a grand spring festival, where the Precure teams from various timelines gather to celebrate. The festival is hosted by a mysterious figure named Announcer, who introduces a series of competitive games and challenges for the Precure teams to participate in. These games are designed to test their teamwork, strength, and magical abilities, creating a lively and festive atmosphere.

As the competition unfolds, the Precure teams face off in various events, showcasing their unique powers and camaraderie. However, the lighthearted festivities take a darker turn when a new antagonist, named Fusion, emerges. Fusion seeks to disrupt the harmony of the festival by absorbing the energy of the Precure teams, aiming to grow stronger and dominate the world. The Precure teams must put aside their competitive spirit and unite to confront this common threat.

The narrative emphasizes the importance of collaboration and friendship across different generations of Precure. Each team contributes their strengths and abilities to combat Fusion, highlighting the diversity and unity of the Precure universe. The film culminates in a climactic battle where the combined efforts of all the Precure teams lead to the defeat of Fusion, restoring peace to the festival and reinforcing the bonds between the heroines.

Key characters include representatives from various Precure series, such as Cure Black and Cure White from "Futari wa Pretty Cure," Cure Dream from "Yes! Pretty Cure 5," and Cure Blossom from "HeartCatch Pretty Cure!" among others. Each character retains their distinct personality and abilities, contributing to the dynamic interactions and teamwork throughout the film. The story balances action, humor, and heartfelt moments, staying true to the themes of the "Pretty Cure" franchise while celebrating its expansive legacy.