"Rilu Rilu Fairilu ~Yōsei no Door~" is a magical girl anime series centered around the lives of Fairilu, tiny fairy-like beings born from magical seeds. Each Fairilu possesses a unique key that unlocks their special abilities and connects them to the mystical Fairilu World. The narrative primarily follows Lip, a cheerful and curious Flower Fairilu, as she navigates her new life after being born from a seed. Lip, along with her friends—including the shy Mermaid Fairilu Sumire, the energetic Insect Fairilu Hana, and the studious Snow Fairilu Yuki—embarks on various adventures to learn about their world and themselves.

The Fairilu World is a vibrant, fantastical realm filled with diverse environments, such as flower fields, underwater kingdoms, and snowy landscapes. Each Fairilu has a distinct role and personality tied to their type, whether they are Flower, Mermaid, Insect, Snow, or other varieties. The series explores their daily lives, friendships, and challenges as they grow and discover their unique powers. A recurring element is the Fairilu Door, a magical portal that connects their world to the human world, allowing for occasional interactions and adventures beyond their realm.

Throughout the series, the Fairilu face various trials, such as helping each other overcome insecurities, solving problems in their world, and occasionally encountering mischievous creatures or misunderstandings. The narrative emphasizes themes of friendship, self-discovery, and cooperation, as the Fairilu learn to work together and embrace their individuality. The episodic structure allows for a mix of lighthearted, whimsical stories and moments of growth, making it accessible for younger audiences while maintaining a sense of wonder and charm. The series also introduces new Fairilu characters over time, expanding the world and its lore.