"Niji-iro Prism Girl" centers around a young girl named Yū Naruse, who dreams of becoming a top idol despite her lack of confidence and experience. The story begins with Yū encountering a mysterious alien named Pico, who crash-lands on Earth and seeks her help to restore his spaceship. In exchange for her assistance, Pico grants Yū the ability to transform into a magical girl, enabling her to perform dazzling idol performances. This newfound power allows Yū to pursue her dream of becoming an idol while also navigating the challenges of balancing her ordinary life with her magical responsibilities.

As Yū begins her journey, she faces various obstacles, including rival idols, self-doubt, and the pressure of maintaining her secret identity. Along the way, she forms friendships with other aspiring idols, such as the confident and talented Rikka Himekawa and the shy but determined Aoi Tōdō. Together, they compete in idol competitions, support one another, and grow both as performers and individuals. The series explores themes of perseverance, friendship, and self-discovery as Yū and her friends strive to achieve their dreams.

The narrative also delves into the comedic and fantastical elements introduced by Pico and his alien technology, which often leads to humorous and unpredictable situations. As Yū gains more experience as an idol, she learns to harness her magical abilities more effectively, using them to overcome challenges and inspire others. The series balances lighthearted moments with emotional depth, highlighting the struggles and triumphs of pursuing one’s passions in a competitive world. Through its blend of idol culture, magical girl tropes, and character-driven storytelling, "Niji-iro Prism Girl" offers an engaging and heartfelt exploration of ambition and camaraderie.