Siobhan, a dedicated PhD student focused on studying the intricate behaviors of deep-sea fauna, embarks on a journey aboard the fishing trawler Niamh Cinn Óir. The vessel is operated by a tight-knit crew, including Captain Freya and her husband, Gerard, who are grappling with the profound loss of their daughter, Niamh. Among the crew are ship engineer Omid Hamilton Accie, as well as Johnny, Ciara, and Sudi Hamilton Accie, all prepared for a fishing expedition. However, their adventure takes an ominous turn when the Irish Coast Guard informs them that their intended fishing location lies within an exclusion zone. Ignoring Freya's orders, Gerard decides to venture there anyway, desperate for a substantial haul to sustain the ship.
Upon entering the exclusion zone, the crew runs into an unfamiliar area with a mysterious object resembling a shoal, causing the boat to halt unexpectedly. Omid uncovers strange breaches in the hull that leak a blue-green slime, which Gerard misidentifies as barnacles. Equipped with diving gear for her research, Siobhan is tasked by Gerard to investigate and free the boat. Beneath the waves, she discovers that the attachment is not barnacles but rather the tentacles of a colossal bioluminescent organism lurking below. Gerard speculates that this may be an undiscovered type of giant squid, leading them to attempt capture with their nets. However, as they attempt to do so, the machinery accidentally catches Johnny's sleeve, resulting in a severe injury contaminated by the mysterious slime.
In search of assistance, Siobhan, Gerard, and Johnny row towards another ship sighted on the horizon. Onboard, they encounter a horrifying scene—bodies of the crew strewn about, with evidence of apparent suicides and one man who seems to have had his eyes gruesomely removed. Gerard rationalizes their death as a manifestation of "sea fever," urging Siobhan and Johnny to keep this discovery from the rest of the crew. Returning to the Niamh Cinn Óir, they find that the creature has released the boat, leading to a fruitful catch that temporarily lifts their spirits. As Siobhan and Johnny share flirtations and explore their mutual attraction, Johnny falls ill, and Siobhan suspects it may be due to an infection but he is reluctant to seek help for his injured hand.
That evening, during a night of celebration, Johnny decides to take a swim but is halted by Ciara and Siobhan. Alarmed as he begins rubbing his eyes, Siobhan checks his pupils with a flashlight, only to witness something alarming moving within them. As he washes his face, Johnny is met with catastrophic blindness; his eyes violently rupture, unleashing small organisms into the ship's water supply, leading to his untimely demise. Meanwhile, Omid realizes that Sudi has joined them in the shower. Despite Omid's attempts to warn him, Sudi continues unaware until the parasitic creatures bite him, resulting in another casualty. In a frantic effort to contain the situation, Gerard, Siobhan, and Omid investigate their water filtration system only to discover it is compromised, with creatures having devoured all the filters. Siobhan deduces that these organisms are larvae from the massive sea creature, which has littered the slime all over the hull.
Desperate, Siobhan experiments with a slime sample, attempting to eliminate the larvae using UV light but to no avail. Instead, with Omid's assistance, she electrifies the trawler using the arc welder, which seems to purify the water supply. Insisting on a self-quarantine for a minimum of 36 hours to prevent any potential infections from spreading, Siobhan's plea is met with dissent, as some argue that Sudi requires medical attention. Tragically, Sudi succumbs to his infection. In an act of desperation, Siobhan immobilizes the boat by entangling the propeller.
As tensions rise, the crew checks one another for any signs of infection. Siobhan, Omid, Freya, and Ciara appear to be free of parasites, but in a devastating twist, Freya discovers that Gerard is infected. The couple shares a poignant farewell before Gerard takes his own life, leaving everyone in shock. As Siobhan begins to suspect Ciara may be infected too, she climbs up to warn the others, only for Ciara to attempt to stop her. In the ensuing chaos, Ciara falls to her death.
In a harrowing decision, Freya chooses to leave the group alone by rowing away despite protests from Siobhan and Omid. In a further exploration of the vessel, Siobhan and Omid discover that the last surviving larva has expanded, compromising the hull and allowing it to escape to the ocean depths. As the boat begins to sink, Siobhan and Omid resort to setting the deck ablaze using white spirit. With an inflatable raft as her only vessel, Siobhan plunges into the water to confront the bioluminescent creature, while Omid, unable to swim, falls into the water, where he is ensnared by tentacles. Siobhan bravely rescues him, but not without suffering a wrist injury from a tentacle. Aware that she has likely become infected, she dives into the depths, aiming for the luminous creature, while Omid is left on the raft, awaiting rescue as help approaches in the distance.
