"Poppen-sensei to Kaerazu no Numa" is a supernatural mystery anime that revolves around the enigmatic figure of Poppen-sensei, a teacher who possesses the ability to communicate with the dead. The story is set in a small, eerie town surrounded by a swamp known as the "Kaerazu no Numa" (Swamp of No Return). The swamp is infamous for its dark history and the numerous disappearances that have occurred there over the years. Poppen-sensei, with his unique gift, becomes involved in investigating these mysterious events, often uncovering unsettling truths about the town and its inhabitants.

The narrative unfolds through a series of interconnected episodes, each focusing on a different case or mystery tied to the swamp. Poppen-sensei's investigations frequently bring him into contact with the spirits of those who have vanished, as well as the living residents of the town who are often hiding their own secrets. His interactions with these characters reveal layers of guilt, regret, and unresolved emotions tied to the swamp's cursed reputation. The series delves into themes of loss, redemption, and the lingering impact of the past on the present.

The main characters include Poppen-sensei himself, a calm and observant figure who serves as both a guide and a mediator between the living and the dead. His presence often brings closure to those affected by the swamp's tragedies, though not always in ways they expect. Other key characters include townspeople who are directly or indirectly connected to the disappearances, each with their own motivations and hidden truths. The swamp itself functions almost as a character, its ominous presence casting a shadow over the town and influencing the events that unfold.

The plot is driven by a combination of supernatural elements and human drama, with each episode building on the overarching mystery of the swamp and its connection to the town. The series maintains a tense and atmospheric tone, blending moments of introspection with eerie and suspenseful sequences. Through its exploration of the swamp's dark history and the personal stories of those affected by it, "Poppen-sensei to Kaerazu no Numa" presents a compelling narrative that intertwines the supernatural with the emotional struggles of its characters.