Natsume Takashi is a teenage boy who has the ability to see spirits, a trait inherited from his grandmother, Reiko. This ability has isolated him from others, as his experiences with spirits often lead to misunderstandings and alienation. After inheriting Reiko's "Book of Friends," a notebook containing the names of spirits she defeated and bound to her will, Natsume becomes the target of both spirits seeking to reclaim their names and those who wish to exploit the book's power.

The series revolves around Natsume's journey to return the names in the Book of Friends to their rightful owners, freeing the spirits from their contracts. Accompanied by Madara, a powerful spirit in the form of a lucky cat (often called Nyanko-sensei), Natsume navigates the delicate balance between the human and spirit worlds. While Nyanko-sensei initially agrees to protect Natsume in exchange for inheriting the book after his death, their relationship evolves into a genuine bond of friendship and mutual respect.

Each episode typically features self-contained stories, often focusing on Natsume's encounters with various spirits. These encounters explore themes of loneliness, connection, and the consequences of human and spirit interactions. Spirits in the series are portrayed with depth, ranging from mischievous and playful to tragic and vengeful, reflecting the complexity of their emotions and histories. Natsume's compassionate nature drives him to help spirits resolve their grievances, even when it puts him in danger.

The human characters, such as Natsume's foster parents, the Fujiwaras, and his classmates, provide a grounding presence in his life. While they are unaware of his ability to see spirits, their kindness and support help Natsume gradually overcome his feelings of isolation. Friends like Tanuma and Taki, who have their own connections to the spirit world, offer understanding and camaraderie, further enriching his experiences.

The series balances episodic storytelling with overarching character development, particularly in Natsume's growth from a lonely, guarded individual to someone who learns to trust and rely on others. The Book of Friends serves as a narrative device to explore the interplay between humans and spirits, as well as the legacy of Reiko, whose past actions continue to shape Natsume's present. Through its gentle pacing and heartfelt storytelling, the series emphasizes themes of empathy, forgiveness, and the importance of human connections.