A small, isolated seaside town holds a dark annual tradition known as Shinjū, a ritual where the community offers a pair of lovers to the raging sea to ensure a bountiful harvest. The protagonist, Kaito, a quiet and observant high school student, has spent his life as an outsider, watching this ceremony from the edge of the crowd. His world collides with the ritual when Shiori, the shrine maiden chosen to perform the ceremonial rites, is unexpectedly forced into the role of the sacrifice. Unlike the tradition's silent acceptance, Shiori refuses to die quietly.

Their escape into the mainland sets the core conflict in motion. The townsfolk, led by Shiori’s father, the village chief, and a mysterious sect of guardians, pursue them to maintain the town’s sacred pact. The narrative explores whether their budding connection is a genuine bond or merely a byproduct of survival instinct against a shared threat. As they flee from the rural coast to the anonymity of the city, the supernatural elements of the Shinjū become undeniable. Shiori discovers she is physically tethered to the sea’s deity, experiencing physical pain the further she travels from the shore, while Kaito begins to see the restless ghosts of previous sacrificed couples, who offer cryptic warnings.

Notable narrative arcs include The Binding, where the pair hides in an abandoned shrine and Kaito learns the true history of the ritual’s origin during a feudal famine. The City Arc follows their attempt to expose the town’s secret to the media, only to find that the guardian sect has long infiltrated metropolitan institutions, turning the city into another kind of cage. Finally, The Return arc forces Kaito and Shiori to confront the deity itself at the bottom of the sea, where they must negotiate a new contract to end the cycle of sacrifice, a journey that tests whether their love is strong enough to break a divine curse.