To pay off a debt owed to Belize crime lord Tariq, Jackson Slate pairs with a gang to descend to the bottom of the Blue Hole in search of rumored Mayan gold hidden in the wreck of the Cortez. They rig the seabed with explosive charges, hoping to unearth wealth, but their reckless blast awakens a prehistoric sea creature that wreaks havoc on the dive team, killing several and leaving [Jackson Slate] unconscious amid the wreckage.
A vacationing couple, Rod and Jane, arrive on the island and set out for a snorkeling excursion after a warm but wary welcome from the boat’s captain, Henry. Jane feels uneasy about the adventure, sensing danger behind the promise of a perfect day. Onshore, Jane endures another awkward encounter with two men who previously harass her, a tension that threads through the morning. When Henry discovers the fallen divers and [Jackson Slate]’s limp form, he carries him back to the mainland for medical help, while the others back at sea piece together the flurry of events. [Jackson Slate] later recounts the terrifying dive to the others, and a plan forms to return to the water to uncover the truth behind the dead and the possible treasure, even as Jane chooses to stay behind, not ready to join.
Back at the Blue Hole, the Belize Coast Guard boards the boat to check credentials and motive, but the divers—[Jackson Slate], Henry, and Dr. Sarah—persuade the authorities to let them resume the expedition for scientific inquiry, arguing that the egg-like clues and tracks they’ve found could illuminate a remarkable discovery. Jane, still ashore, finds herself confronted again by the same two men, who press her to go partying on their boat. Meanwhile, the underwater team discovers dinosaur tracks and a clutch of eggs on the seabed, and they carefully salvage one egg for laboratory study.
As the team studies the egg, a baby creature hatches, and the trio—[Jackson Slate] and Dr. Sarah—narrowly trap the newborn in a freezer to keep it contained. The moment is interrupted when Tariq and his men arrive, forcing [Sarah] at gunpoint and demanding the recovered gold. In a tense gambit, [Jackson Slate] tricks one of Tariq’s guards into releasing the freezer, and the infant dinosaur launches a furious attack, allowing the scientists to escape as chaos erupts around them.
The party boat Poseidon becomes a nightmare as the monstrous creature, now known as Poseidon Rex, assaults the watercraft. Jane is knocked into the sea and loses her life, while the creature continues its rampage, slaughtering everyone aboard. The survivors—[Jackson Slate], [Rod], and Dr. Sarah—make their way back to the Blue Hole with Coast Guard support to reexamine the eggs, but another Rex assault claims Henry’s life and swallows the Coast Guard boat as the beast emerges toward the mainland.
On land, the trio collide with Tariq again, only to witness the Rex turn on him, swallowing the crime lord in a gruesome moment that underscores the creature’s indiscriminate power. After a fragile, hard-won moment of romance between [Jackson Slate] and [Sarah], the remaining allies fortify Tariq’s hideout and learn of an impending military strike designed to wipe out the surviving creature.
A final chase ensues as the heroes pursue a plan to stop the monster once and for all. [Jackson Slate] pilots a small plane to create a distraction while [Sarah] and [Rod] escape by boat. Tragically, [Rod] is killed when he tumbles into the water after firing a bazooka at the Rex. The Army’s assault ultimately fails to contain the beast, but [Sarah] seizes a rocket launcher and delivers a decisive blow, blowing the creature’s head off. In the aftermath, [Sarah] reunites with [Jackson Slate] at the pier, marking a bittersweet end that blends loss with a hard-won sense of hope for a safer future.
