A small, secluded island off the coast of Belize suddenly finds itself terrorized by a deadly predator from the planet's distant past when deep sea divers accidentally awaken an ancient evil.
Forced to pay off his debt to the murderous Caribbean crime lord, Tariq, instead, the American scuba-diver and treasure hunter, Jackson Slate, wakes up from its deep hibernation a massive, T-Rex-like sauroid off the coast of Belize. Now, Slate has to team up with diver, Henry, happy holidaymakers, Rod and Jane, and the marine biologist, Sarah Jones, to retrieve a Mayan treasure from the dangerous waters; however, the scaly beast from the sea is ever-present and famished. But, what good is a chest full of gold when you're dead? Can Jackson and his team defeat the all-powerful reptile, Poseidon Rex?—Nick Riganas
To pay off a debt owed to Belize crime lord Tariq, Jackson Slate goes with several gangsters on a dive to the bottom of the Blue Hole in search of rumored Mayan gold in the wreck of the Cortez. Using explosive charges on the sea floor, the men inadvertently unearth a prehistoric sea creature that attacks the dive crew killing all and leavings Jacks floating unconscious.
Vacationing couple Rod and Jane arrive at the Island. Walking to the boat to go snorkeling, Jane gets hit on by two guys in front of Rod. At the boat captain Henry tells them about the wonderful snorkeling experience over the Blue Hole. Jane is not happy that it could be dangerous. Snorkeling they find Jacks' unconscious body. Their boat captain Henry takes Jacks back to the mainland where he is treated by a scientist named Sarah. Jacks tells' everyone what happened, and they all return to the sea to investigate, except for Jane who stays behind again not happy with Rod.
At the Blue Hole, the Belize Coast Guard boards the boat and hassles the group. Jacks, Henry, and Sarah convince the Coast Guard to let them dive and investigate. Jane lying out on the beach gets approached again by the two men who hit oh her earlier. They tell her to come with them and party on their boat.
Jacks, Sarah and Henry dive while Rod chickens out and stays in the boat. At the bottom, they find dinosaur tracks. They also find eggs. They retrieve an egg, and take it back to a lab for research.
Jacks and Sarah examine the egg and a baby creature hatches. Jacks and Sarah trap the creature in a freezer when it attacks. Tariq and his men arrive, taking Sarah at gunpoint and demanding that Jacks turn over the gold. Jacks tricks one of Tariq's men into opening the freezer. When the baby dinosaur attacks, Jacks and Sarah escape.
On the party boat Poseidon Rex attacks. Janes gets knocked into water and dies. Poseidon Rex attacks and kills everyone on the boat. Rod, Henry, Jacks & Sarah return to the Blue Hole with the Coast Guard to reexamine the eggs and Henry dies during another Rex attack. The coast guard boat is also swallowed before Poseidon Rex rises to attack the mainland.
Jacks, Rod, and Sarah run into Tariq again back on land, but Rex suddenly eats Tariq. After Jacks and Sarah share a romantic moment, the remaining trio stocks up on weapons at Tariq's hideout and learns of an impending Army attack on the island to destroy the creature.
Following another pursuit and fight with Rex, Jacks flies a small plane as a distraction while Sarah and Rod flee by boat. Rod falls in the water after firing a bazooka at Rex and he dies. After the Army's attack fails, Sarah recovers the rocket launcher and blows the head off Poseidon Rex. In the aftermath, Sarah reunites with Jacks at the pier.
