Siobhan, a scientist studying fauna behavioral patterns, purchases a place on the fishing trawler the Niamh Cinn-Oir, manned by a crew of six: skipper Gerard, his wife Freya, ship's engineer Omid, Johnny, Ciara, and Sudi. As they set off, they are warned by the Coast Guard that there is an exclusion zone near their route, which Gerard, unbeknownst to the others, takes them into. As they travel through the zone, they run into an unknown, shoal-like object which stops the boat. Meanwhile Omid discovers strange breaches in the hull which protrude a blue-green slime. The crew suspects barnacles, and as Siobhan possesses the only diving gear on-board, Gerard sends her armed with a knife to cut the boat free. Underwater, she discovers tendril-like appendages of an enormous organism adhering to the boat. Thinking it may be a newly-discovered form of giant squid, the crew attempt to capture it with their net, but it proves too heavy. While releasing the net, Johnny cuts his hand on the rope.Seeing another boat nearby, Gerard, Johnny, and Siobhan board it using a rowboat, only to discover the entire crew dead from suicide inside the cabin, with one of the men having his eyes ostensibly scooped out. They return, with Gerard instructing the others not to speak of it, calling it "sea fever" and insisting the other crew merely went mad. That night, Johnny begins to exhibit uncharacteristic behavior, including wanting to go for a swim. Suspecting a fever, Siobhan performs a medical diagnosis on him and discovers movement in his retinas. As he is washing his face, he suddenly goes blind and his eyes burst, releasing minuscule organisms. Siobhan realizes they are parasites that were attempting to go into the water, and the crew rushes to save Sudi, who is taking a shower. Sudi suffers numerous cuts over his body as the waterborne creatures bite him, and Johnny dies from his injuries.Siobhan theorizes that the underwater creature, mistaking the trawler for a whale, latched onto the boat to inject its larvae in the form of the slime. Johnny, who had a cut on his hand, was infected by the parasitic creatures, and the rest of the crew are also at risk. The crew check their water filtration system and find that their entire water supply has been contaminated. As the creatures are a hadopelagic (below the abyssopelagic layer) species, Siobhan theorizes intense UV light may harm it and they attempt to use it to kill the microscopic eggs in the slime. When that fails, they instead electrocute the trawler using the ship's arc welder, successfully sterilizing the boat and preventing further infection. However, this does not preclude the chance that they may have already been infected, and Siobhan insists they self-quarantine on the trawler for a further 36 hours before returning to shore. When Sudi dies from the infection, Siobhan disables the trawler by entangling the propeller.The crew diagnose themselves by checking their eyes for the parasites, and Siobhan, Omid, Freya, and Ciara ostensibly check out. Gerard however, is proven to be infected, and after a heartfelt goodbye Freya mercy kills him by slicing his throat. Ciara goes insane and attacks Siobhan, only to fall to her death. As she dies, a parasite emerges from her eye, revealing her to have been infected. Now with only Freya, Siobhan, and Omid remaining, Freya decides to take the rowboat to shore to double their chances of being found. Siobhan and Omid discover that the mass of larvae in the ship's water tank have instead now been replaced by a single cannibal creature, which eats through the ship's hull to return to the ocean, causing the trawler to take water and sink. They use spirits to ignite a fire on the Niamh Cinn-Oir and then abandon it on the inflatable boat, but Omid falls overboard and is seized by the adult creature. Siobhan rescues him, but cuts her bare hand on the creature's tendril, becoming infected. As a rescue helicopter approaches, Siobhan dives into the water, swimming towards the bio-luminescent creature.