Tiko Komori is an eleven-year-old Japanese schoolgirl defined by her initial shyness and introverted demeanor. She receives a mysterious birthday gift from her father—a book that releases the mischievous fairy Tickle upon opening, freed after being imprisoned for pranking humans. Skeptical of Tickle's magical claims, Tiko demands proof; Tickle responds by instantly delivering a handmade scarf to Tiko's distant friend Miko, convincing Tiko to befriend her.

To conceal Tickle within her household, Tickle magically reshapes the memories of Tiko's parents and younger sister Hina, embedding herself as Tiko's lifelong twin sister. This deception creates a dual existence where Tickle masquerades as family while covertly wielding magic. Tiko consistently opposes Tickle's magical pranks, especially those targeting Hina, highlighting her ethical unease with supernatural manipulation.

Despite her reserved nature, Tiko cultivates growing confidence and assertiveness through her bond with Tickle. Her loyalty manifests in daily life as she navigates being Tickle’s confidant while tempering her trickster impulses. Tiko’s experiences intertwine ordinary childhood trials—school conflicts, family bonds—with Tickle’s magic, which alternately disrupts or resolves these challenges.

The narrative culminates when Tickle exposes her powers to prevent their mother’s accident, breaching the secrecy preserving her place in the family. This revelation triggers consequences: Hina, consumed by distress, burns the book serving as Tickle’s gateway to her magical realm. Tickle chooses permanence with the Komoris, surrendering her magic. Tiko’s acceptance of this sacrifice underscores their profound connection, concluding their transformative journey together.