"Hotaru" centers on the life of a young girl named Hotaru, who moves to a secluded village to live with her grandparents after her parents' untimely death. The village, surrounded by dense forests and steeped in folklore, harbors secrets tied to ancient traditions and supernatural phenomena. Hotaru, initially struggling with grief and isolation, begins to uncover the village's mysterious past through her interactions with its peculiar inhabitants.

The narrative unfolds as Hotaru befriends a boy named Kaito, who is deeply connected to the village's legends. Kaito reveals that the village is protected by a guardian spirit, which manifests as a glowing firefly. This spirit, tied to the villagers' ancestors, is said to maintain the balance between the human world and the spirit realm. However, the spirit's power is waning, and the boundary between worlds is weakening, leading to strange occurrences and the appearance of malevolent entities.

As Hotaru delves deeper, she learns that her family has a unique connection to the guardian spirit. Her grandmother, a revered figure in the village, possesses knowledge of ancient rituals necessary to restore the spirit's strength. Hotaru, guided by her grandmother and Kaito, embarks on a journey to gather sacred artifacts scattered across the forest. These artifacts, tied to the village's history, hold the key to reinforcing the barrier between worlds.

The story explores themes of loss, resilience, and the interplay between tradition and modernity. Hotaru's growth is central to the narrative, as she transitions from a grieving outsider to a pivotal figure in preserving the village's legacy. Kaito serves as both a guide and a foil, his unwavering belief in the village's traditions contrasting with Hotaru's initial skepticism. The grandmother, a stoic yet compassionate figure, bridges the gap between the past and present, offering wisdom and support.

The plot intensifies as the malevolent entities grow stronger, threatening both the village and the spirit realm. Hotaru and Kaito face numerous challenges, including deciphering cryptic clues, confronting their fears, and navigating the treacherous forest. The climax involves a ritual to restore the guardian spirit, requiring Hotaru to confront her deepest emotions and embrace her role in the village's destiny.

The series concludes with the barrier restored, but not without sacrifices. Hotaru's journey leaves her with a renewed sense of purpose and a deeper connection to her heritage. The village, though forever changed, continues to thrive under the protection of the guardian spirit, symbolizing the enduring power of tradition and community.