Shigofumi are letters from the dead, written by those who have recently passed away and permitted to send a final message to the living. These letters can contain anything the deceased could not say in life, from confessions of love to final goodbyes, and even the identity of their murderer. The delivery of these potentially devastating messages falls to special mail carriers who reside in a realm called Shigo when not working.

The story follows Fumika, a quiet and eerily composed young girl who serves as one of these carriers. Accompanied by her talking staff, a loud and boisterous personality named Kanaka, she delivers shigofumi to recipients in a Japanese city. Unlike other carriers who are already deceased and do not age, Fumika continues to grow older, a mystery that hints at her unusual circumstances. Her calm demeanor masks a deep-seated trauma; she suffers from dissociative identity disorder resulting from years of abuse by her father, Kirameki Mikawa, a famous writer obsessed with beauty. One of her personalities, Mika, is the stoic mail carrier, while the other, Fumi, is a kind and gentle girl who fell into a coma-like state after Mika shot their father in self-defense.

The narrative follows a largely episodic structure as Fumika and Kanaka deliver shigofumi to various recipients. Each delivery forces both the sender and receiver to confront hidden truths, often leading to dramatic and tragic consequences. One early arc involves a high school student named Shota Machiya, an enthusiast of model rocketry, and his friend Asuna. Fumika delivers a shigofumi to Asuna from her deceased father, a letter that carries shocking revelations about his death and his true feelings. Another significant character is Kaname Nojima, a boy who recognizes Fumika from their junior high school days. He remembers the incident three years prior when she shot her father and becomes determined to uncover the truth about her past, a search that forms a central mystery throughout the series.

As the light novel progresses, Fumika's own history becomes increasingly intertwined with her deliveries. The line between her duty as a carrier and her unresolved personal trauma blurs, forcing her to confront the fragmented pieces of her identity and the fate of her alternate personality, Fumi. The four volumes collect these stories, exploring themes of death, regret, communication, and the painful but necessary act of facing the truth left behind by those who have died. The story in the light novels shares the same premise as the anime adaptation but features notable differences, such as Fumika’s phobia of insects instead of cats, her talking staff being a young boy named Mayama with a different personality, and her fondness for playing shogi.