"Dark Sea, Moon Shadow" is a supernatural mystery anime set in a coastal town shrouded in folklore and eerie phenomena. The narrative centers on Haruto, a reserved high school student who returns to his ancestral home after the sudden disappearance of his older sister, Akari. The town is infamous for its legends of the "Dark Sea," a cursed expanse of water said to claim those who venture too close during the new moon. Akari’s vanishing coincides with the appearance of strange occurrences, including ghostly apparitions and unexplained tides, reigniting local fears of the curse.

Haruto’s investigation leads him to uncover a hidden connection between his family and the Dark Sea. His grandmother, a reclusive woman with deep knowledge of the town’s history, reveals that their lineage is tied to an ancient pact with the sea spirits. This pact, meant to protect the town, has begun to unravel, threatening to unleash chaos. Haruto learns that Akari’s disappearance is linked to her role as the "Moon Shadow," a chosen individual destined to maintain the balance between the human world and the spirit realm.

As Haruto delves deeper, he encounters Yuna, a mysterious girl who claims to have been born from the Dark Sea itself. Yuna possesses fragmented memories of the curse and becomes a key ally in Haruto’s quest. Together, they navigate a labyrinth of cryptic clues, encountering otherworldly beings and unraveling the truth behind the curse. The story explores themes of sacrifice, familial duty, and the blurred boundaries between the living and the supernatural.

The plot intensifies as Haruto discovers that the curse’s resurgence is tied to a forgotten betrayal by his ancestors, who broke the pact to gain power. To save Akari and the town, Haruto must confront the spirits of the Dark Sea and restore the balance, even if it means risking his own life. The series culminates in a climactic confrontation during the new moon, where Haruto must make a pivotal choice that will determine the fate of both worlds. The narrative weaves together elements of suspense, folklore, and emotional depth, creating a compelling tale of mystery and redemption.