"Mōryō no Hako: Hako no Yūrei no Koto" is a supernatural mystery anime set in post-World War II Japan, blending elements of horror, psychological intrigue, and folklore. The narrative revolves around a series of bizarre and interconnected events tied to a mysterious box and its supernatural influence. The story begins with the discovery of a dismembered body in a box, which draws the attention of several key characters, including a detective, a novelist, and a medical student, each with their own motivations and connections to the case.

The central figure, Sekiguchi, is a medical student who becomes entangled in the investigation after encountering the enigmatic novelist, Kiba. Kiba, who possesses a deep interest in the occult and the supernatural, becomes a pivotal figure in unraveling the mystery. Their paths cross with Detective Kiba, a pragmatic investigator who is initially skeptical of the supernatural elements but is forced to confront the inexplicable as the case deepens. The narrative also introduces Kanako, a young girl with a mysterious connection to the box, whose presence adds layers of complexity to the unfolding events.

As the investigation progresses, the story delves into themes of human psychology, the nature of evil, and the blurred lines between reality and the supernatural. The box, believed to be cursed, serves as a catalyst for the characters' descent into a world of paranoia, obsession, and violence. The narrative structure is non-linear, with flashbacks and shifting perspectives that gradually reveal the connections between the characters and the box's dark history. The series explores the impact of trauma, the fragility of the human mind, and the consequences of meddling with forces beyond comprehension.

The anime's atmosphere is heavily influenced by Japanese folklore and post-war societal tensions, creating a sense of unease and foreboding. The characters' interactions and personal struggles are interwoven with the supernatural elements, making the story both a psychological thriller and a horror tale. The resolution of the mystery ties together the disparate threads of the narrative, culminating in a chilling exploration of the human condition and the lingering effects of unresolved trauma.