Sea Fever

Siobhan joins the crew of the Niamh Cinn Óir, a fishing trawler, along with Captain Freya, Gerard, Omid, Johnny, Ciara, and Sudi. They set out on a fishing trip despite warnings from the Irish Coast Guard about entering an exclusion zone. Gerard decides to ignore the orders, needing a large haul of fish to make the trip profitable.

While in the exclusion zone, the ship collides with a mysterious, shoal-like object. Omid notices strange breaches in the hull emitting blue-green slime, which Gerard dismisses as barnacles. This marks the beginning of their encounter with an unforeseen marine anomaly.

Gerard sends Siobhan underwater equipped with diving gear to cut the boat free from the creature. She observes enormous bioluminescent tentacles attached to the ship, which she initially suspects to be a newly discovered giant squid. Attempts to capture the creature with a net fail as it is too heavy.

Siobhan, Gerard, and Johnny notice another ship nearby on the horizon. They row toward it for help and find its crew dead, with signs of apparent suicide and one man missing his eyes. Gerard concludes they likely succumbed to 'sea fever' and instructs others not to reveal this to the main crew.

After returning to their ship, the crew manages to lower nets and haul up a substantial catch of fish, boosting morale. Siobhan and Johnny share a flirtatious moment, hinting at their growing connection amid the tense atmosphere.

Johnny declares he will go for a swim, but Siobhan notices him rubbing his eyes and later finds his eyes moving abnormally. His condition worsens rapidly, and he goes blind as his eyeballs burst, releasing tiny organisms into the ship's water system, which results in his death.

Omid and Siobhan discover the ship's water filtration system is infected by the organisms, which have eaten through the filters. They realize the creatures are larvae of the enormous sea creature, whose eggs are spreading across the ship, threatening everyone on board.

Siobhan conducts experiments and unsuccessfully tries to kill the larvae with UV light. She then electrifies the ship with an arc welder to sterilize the water, ordering others to quarantine themselves for 36 hours to prevent infection spread.

The crew examines each other's eyes for parasites. Siobhan, Omid, Freya, and Ciara initially seem clear, but Freya detects parasites in Gerard's eyes. Gerard then kills himself to prevent infecting others, revealing the severity of the outbreak.

Siobhan uncovers that Ciara is infected and attempts to alert others, but she is pushed off the ladder and falls to her death. The infection kills Ciara as the ship's situation deteriorates rapidly.

Despite protests, Freya escapes alone in a rowing boat, leaving the increasingly unstable crew behind. The others discover that the remaining larva has grown larger and eaten its way through the hull, threatening to sink the vessel.

Siobhan and Omid set the deck on fire in a last effort to destroy the creature and prevent it from escaping. Siobhan boards an inflatable raft, witnessing Omid being pulled underwater by tentacles as she prepares for her final struggle.

After cutting her wrist to infect herself, Siobhan swims toward the bioluminescent creature to destroy it from within. She hopes that her sacrifice will prevent the organism from spreading further into the ocean.

From the raft, Omid watches as distant help approaches, leaving the fate of Siobhan and the ocean creature uncertain. His rescue marks the ambiguous end to their dire encounter with the deadly marine organism.