Natsuki Koyama is a lively fifth-grade student adjusting to countryside life in Murakawa village after moving from town. She lives with her aunt's family and her younger cousin, Amane Suzuki, attending a tiny elementary school with just five students. Natsuki bursts with energy and a tomboyish spirit, chasing fun with a direct and sometimes unrestrained approach. This enthusiasm occasionally leads to clumsy social moments, like her habit of petting animals upwards from the lower back—a move consistently met with animal disapproval, even provoking a chase from a large cow-like alien.

Natsuki fiercely yearns to be a hero, craving admiration as a role model, especially from Amane. However, this aspiration clashes with her own immature actions compared to her younger cousin, causing Amane to pull away. The strain worsens when Amane’s rabbit, Pyon-Kichi, vanishes—an escape secretly triggered by Natsuki’s attempt to teach it tricks. Hiding her responsibility fractures their relationship further.

Unknowingly carrying a wasabi root resembling the rare alien energy source Zughan, Natsuki attracts dangerous attention during a space journey. Her heroic ideals face a harsh test when Amane is kidnapped. Confronting real peril, Natsuki feels fear and helplessness, starkly realizing the gap between her fantasies and the daunting reality of rescue, forcing her to confront self-doubt and limitations.

Pushing past her fear, Natsuki joins the rescue during a climactic confrontation at an interstellar festival. This action, combined with the revelation that Neppo’s henchmen—not her—caused Pyon-Kichi's prolonged disappearance, helps reconcile her with Amane. The shared ordeal and her active role demonstrate a shift from seeking admiration towards embodying genuine courage and responsibility.