Captain John Finnegan, [Treat Williams], and his crew—Joey Pantucci, [Kevin J. O'Connor], and Leila, [Una Damon]—are hired by a group of mercenaries, Hanover, Mulligan, Mason, Billy, T-Ray, Mamooli, and Vivo, to pilot their boat across the South China Sea to an undisclosed rendezvous point. At the same time, the Argonautica, a luxury cruise ship built and owned by Simon Canton, [Anthony Heald], is embarking on its maiden voyage. A thief named Trillian St. James, [Famke Janssen], is discovered and locked in the brig, while an unidentified saboteur cripples the ship’s navigation and communication systems. Then something rises from the depths, rams the vessel, and slaughters the passengers, leaving aboard the Argonautica a chilling silence that unsettles everyone.
A speedboat that the Argonautica had shaken loose collides with Finnegan’s boat, and the mercenaries seize control with the aim of looting the passengers, plundering the vault, and then sinking the ship with torpedoes stowed aboard. Finnegan musters resilience with his small crew, but Leila and Billy are left behind to repair their craft as the mercenaries board the ship. They reach the ballroom only to find blood-soaked scenes and no living passengers. Finnegan and Joey head to the workshop to scavenge parts for repairs, while Mamooli and T-Ray guard them. In the chaos, Leila is dragged away by unseen creatures, and T-Ray is dragged off to what sounds like doom. Mamooli is also taken, and the two men flee, encountering an escaped Trillian as they stagger through the wreckage.
Inside the vault, Canton surfaces from hiding and—mistakenly—kills Vivo during the commotion. The mercenaries panic and, in the confusion, kill several passengers. Canton, the saboteur, Captain Atherton, and a few others flee the creatures and come face to face with Finnegan, Joey, and Trillian. The truth begins to surface: Canton is the saboteur who orchestrated the sinking for insurance money. The crew’s fragile alliance unravels as the creatures attack, and a partially digested body, Billy, is revealed after Finnegan shoots at the monster. The charges of fear and greed collide with survival as the creatures close in.
As the survivors run, Canton tries to throw off the others by steering them toward a retreat, while the creatures reveal themselves to be an evolution of Ottoia, draining the victims’ fluids and leaving behind bloody, skeletal remains. The group watches as Mason is grabbed and, in a final act of defiance, detonates a grenade and dies. Mulligan stays behind in the galley, determined to wait for rescue, but a creature corners him. He scorches it with fire from the stove, only to be swallowed from behind by another creature, a grim end to a once-doubtful ally.
The survivors’ world narrows to a brutal corridor of danger as they are herded toward the ship’s bow. Canton, intent on erasing witnesses to his fraud, misleads the others toward a narrow exit while he lopes toward an escape. The bow becomes a feeding ground, a gruesome theatre where skeletal remains litter the decks as the hull tears open and sea creatures flood the lower levels, separating the group. Hanover, who has tried to slow the beasts with whatever weapons are at hand, is eventually devoured, leaving Joey to face the terror alone. In a tense moment of mercy, Joey offers Hanover a gun, only to witness his comrade squander the chance at redemption and die.
Finnegan and Trillian spot a small island in the distance and set their sights on escape, but their engine parts are missing, and their improvised craft is nearly useless. Joey returns to help, and Finnegan programs the autopilot to crash his boat into the Argonautica and detonate the torpedoes, a desperate bid to end the threat. Trillian buckles down and locates a jet ski loaded with fuel on the ship, hoping to reach the island. Canton arrives in pursuit, armed with a flare gun, and threatens Trillian’s fate; she escapes him and dashes away. Finnegan pursues Canton toward the ballroom, determined to rescue Trillian, and the two collide once more amid the wreckage.
The creatures finally reveal themselves as the tentacles of a colossal deep-sea terror—the Octalus—and Finnegan shoots the beast in its eye, blinding it long enough for him and Trillian to flee on the jet ski. Canton makes a last, harebrained bid to seize Finnegan’s ride, leaping onto the boat but breaking his leg. Crippled, he cannot deactivate the autopilot and dies when the rogue craft collides with the Argonautica, detonating the torpedoes and destroying both ships and the Octalus in a final, explosive gambit.
Finnegan and Trillian reach the island and reunite with Joey, who they believed was lost. A distant roar echoes from the jungle, followed by the sight of an active volcano in the distance. As a massive form crashes through the trees toward the beach, the trio faces a new and even more perilous horizon, leaving the fate of their future hanging in the steam and smoke of the approaching eruption.
