As Garda Ciarán O'Shea (Richard Coyle), a man grappling with his troubled past, finds himself in an uneasy partnership with the industrious and driven Garda Lisa Nolan (Ruth Bradley) on a secluded Irish island, his initial dismissiveness toward her eagerness to prove herself is palpable. The serenity of their small community is abruptly disrupted when they make a shocking discovery: mutilated whale carcasses litter the shores, hinting at a sinister reality — they are under siege by a supernatural force. This alien threat, referred to as the "Grabbers," emerges from an eerie green light that has descended upon the island.
A local eccentric, Paddy (Lalor Roddy), miraculously survives an initial encounter with the creatures, leading Dr. Smith (Russell Tovey), a marine ecologist, to theorize that his elevated blood alcohol level rendered him unappetizing to the Grabbers, who thrive on blood and water. However, as O'Shea struggles to communicate with the mainland for help, an impending storm worsens the situation, cutting off any hope for rescue.
Feeling the pressure mount, Nolan and O'Shea decide to organize a gathering at the local pub to keep the residents calm and safe, blissfully unaware of the lurking peril. Hesitant at first, the townsfolk are quickly drawn in by the promise of free drinks from Brian Maher (David Pearse), the pub owner. While the rest indulge in merriment and drinks, O'Shea opts to stay sober, focusing on organizing the town's defenses amidst growing chaos.
As the night unfolds, Nolan unexpectedly opens up to O'Shea about her personal quest to escape the shadow of her more favored sister, leading her to confess her feelings for him, despite their earlier misunderstandings. Meanwhile, an inebriated Smith ventures outside, hoping his drunkenness will protect him while capturing an image of their monstrous foe. Tragically, he becomes the monster's next victim.
Desperately, Nolan and O'Shea race back to the pub, trying to safeguard the townspeople. In her intoxicated state, Nolan attempts to calm the panicking crowd, but her reassurances only incite further fear. As panic escalates, they retreat to the upper floor of the pub, initially unaware that baby Grabbers have infiltrated the ground level. In a frantic bid for safety, Nolan accidentally ignites the pub in flames, but miraculously, she and O'Shea divert the attention of the adult creatures, granting them a short-lived respite.
Their perilous journey continues as they navigate the eerie terrain in their vehicle, relentlessly pursued by the monstrous entity. With hopes of trapping the beast at a construction site, they find themselves in a tense struggle. O'Shea is bravely attacked but remains resilient. Seizing the chance, Nolan utilizes heavy machinery to immobilize the creature at the edge of a gaping pit. Just as the creature regains its grip on O'Shea, he cleverly uses its momentarily loosened hold to douse it with potent Paddy’s poitín, freeing himself and incapacitating the monster.
Seizing the opportunity while the creature is momentarily subdued, Nolan takes aim with a flare gun, igniting nearby explosives that deliver a decisive blow to the Grabber. As the storm begins to wane, O'Shea discards his flask, symbolizing the tragic repercussions of their terrifying encounter. Together, they return to town, blissfully unaware that a new batch of Grabber eggs lies buried on the beach, poised to hatch into an even greater menace.
