"Momiji" is a supernatural drama set in a rural Japanese village surrounded by dense forests and ancient traditions. The narrative centers on Momiji, a young girl with a mysterious connection to the natural world, who becomes the focal point of a centuries-old conflict between humans and spirits. The village is protected by a sacred tree, which is said to house a powerful spirit that maintains the balance between the human and spirit realms. However, the tree begins to wither, and strange occurrences plague the village, signaling an impending catastrophe.

Momiji, orphaned at a young age, is raised by her grandmother, who is the village's spiritual guardian. As the tree weakens, Momiji discovers she possesses the ability to communicate with spirits, a rare gift that marks her as the chosen one to restore balance. Her journey is intertwined with the arrival of Kaito, a wandering scholar with a hidden agenda, and Yuna, a spirited village girl who becomes Momiji's closest friend. Together, they uncover the truth behind the tree's decline: a vengeful spirit, sealed away long ago, is attempting to break free and exact revenge on the village for its past transgressions.

The plot unfolds through a series of episodic encounters with various spirits, each representing different aspects of nature and human emotion. These encounters challenge Momiji's understanding of her role and the moral complexities of the conflict. As the village prepares for an annual festival meant to honor the spirits and strengthen the sacred tree, Momiji must confront her own fears and doubts while navigating the growing tension between the villagers and the spirit world. The climax revolves around a desperate attempt to perform an ancient ritual to save the tree, with Momiji at its center, forcing her to make a profound sacrifice to protect both realms.

The main characters are defined by their relationships to the central conflict. Momiji embodies innocence and determination, serving as a bridge between humans and spirits. Kaito provides a pragmatic perspective, often questioning the village's traditions, while Yuna represents unwavering loyalty and courage. The antagonist, a corrupted spirit named Akuma, symbolizes the consequences of human greed and neglect, driving the narrative's tension and moral dilemmas. The story explores themes of coexistence, responsibility, and the cyclical nature of life and death, all set against the backdrop of a richly detailed supernatural world.