In 1914, a young Irish meteorologist travels to a remote South Atlantic island to work as a weather observer, sharing the outpost with Gruner, Ray Stevenson, the blunt, solitary caretaker of a crudely fortified lighthouse. Gruner explains that the previous observer died of typhus and that there is no body to bury, a detail that unsettles the newcomer as he begins to map the island’s winds and currents.
Exploration leads him to the predecessor’s diary, filled with sketches of strange sea creatures that seem to watch from the waves. At night, the island’s foggy darkness erupts with assaults from those eerie beings, and on the following night, a desperate attempt to repel them with a fire ends with the destruction of his own cabin. The next day, the man finds a female amphibious creature outside, and Gruner blocks any attempt to shoot her, revealing that she is “tame.” Aneris, Aura Garrido, becomes a living presence on the cliffside, and Gruner’s gaze makes it clear that she will not leave her master, no matter how cruel he may be.
The newcomer, now known as Friend, David Oakes, bribes Gruner to allow him to stay in the lighthouse, gradually carving out a fragile routine between isolation and fear. Aneris sings during the nightly attacks, a sound that oddly softens the air around the looming danger, while Gruner dismisses Friend’s more hopeful interpretations as naïve fantasies. The two men drift into a tense cohabitation, each asserting his own view of what protects them both.
As days pass, Friend’s attachment to Aneris grows, and he begins to see her not as a possession but as a creature with a stubborn sense of kinship. He even names her Aneris, and he believes the nightly onslaughts are attempts by the sea to reclaim her, a theory Gruner mocks, urging him to abandon such romantic notions. The lighthouse endures another flood of claws and teeth, and when the building is nearly overrun, Gruner’s brutal pragmatism clashes with Friend’s emerging empathy.
One day, Aneris leads Friend toward a derelict rowing boat, hinting at a possible escape. In response, the two men hatch a plan to salvage dynamite from a sunken ship just offshore. Friend dons an old diving suit and descends to recover several boxes of dynamite, while Gruner prevents any rescue and lowers the air supply, testing whether the other can survive. Returning to the surface, Friend clambers back into the rowboat, keenly aware that Gruner has again tried to end him. Yet the dynamite is dry enough to use, and the two men conjure a gambit to lure the sea’s creatures to the lighthouse before detonating the charges.
The blast devastates many attackers, and the morning after finds Gruner finishing off the remaining beasts. Friend retrieves a necklace from one of the dead creatures and presses Gruner to acknowledge that these beings may be more civilized than he believes. The true turn arrives when Gruner reveals he had found Aneris trapped in his net at sea and freed her, a revelation that shifts the balance of power and affection between the three of them. Aneris’ absence after that night—a mournful wail from the sea—signals a turning point: she reappears, now more confident, while a child creature is discovered nearby.
Friend attempts to broker peace, showing Aneris a token of hope, but Gruner commands Aneris to return to him, and she refuses. The two men fight again inside the lighthouse, the air thick with anger and fear. Friend pleads for Gruner to remember love, and in a final, fateful moment, Gruner drops his axe and walks out of the lighthouse, where he is sealed by a surrounding mob of sealing creatures.
Months later, a replacement ship sweeps in, its captain announcing arrangements to revoke the weather observer position. Friendly deception is kept alive as Friend tells the captain that the weather observer died of typhus, echoing the lie he had been told upon arrival. Aneris watches from a distance, then takes one last leap into the sea, swimming away as the island’s walls echo with both loss and a fragile, unsettled peace.
