"Devil in the Water" is a manga series that blends elements of mystery, supernatural horror, and psychological drama. The narrative centers around a small, isolated coastal town plagued by a series of unexplained drownings. The drownings are attributed to a malevolent entity believed to reside in the nearby waters, referred to locally as the "Devil in the Water." The story unfolds as a young outsider, Haruto, arrives in the town to investigate the phenomenon after his sister becomes one of the victims. Haruto’s skepticism about the supernatural is challenged as he uncovers the town’s dark history and its connection to the entity.

The town’s residents are deeply superstitious, with many believing the drownings are a curse tied to an ancient ritual gone wrong. Haruto encounters a range of characters, including Aoi, a local fisherman’s daughter who has lost multiple family members to the drownings, and Renji, a reclusive historian who holds cryptic knowledge about the town’s past. As Haruto delves deeper, he discovers that the entity is not merely a myth but a manifestation of collective guilt and unresolved trauma tied to a tragic event decades earlier. The drownings are revealed to be selective, targeting individuals with hidden connections to the town’s history.

The plot intensifies as Haruto and Aoi uncover a pattern linking the drownings to specific dates and locations, suggesting the entity operates under a set of rules. Their investigation leads them to confront the town’s elders, who have long suppressed the truth to maintain order. The climax involves a ritual to appease the entity, forcing Haruto to make a difficult choice between saving the town and preserving his own moral integrity. The series explores themes of guilt, redemption, and the consequences of burying the past, with the entity serving as both a literal and metaphorical force of retribution. The story concludes ambiguously, leaving the fate of the town and its inhabitants open to interpretation.