This is the summary of the movie.
Jonas Taylor, a rescue diver, attempts to save a group of scientists, among them Dr. Heller, from a massive, underwater shark attack. Taylor is able to rescue some but refuses to go back to save the rest, having realized that going back for the rest would mean the death of all.
Five years later, Jack Morris meets Dr. Minway Zhang at an underwater research facility called "Mana One", which Morris finances. Zhang and his daughter, Suyin, an oceanographer, are supervising a mission to dive further into the depths of the ocean, which is being conducted by Lori, Taylor’s ex-wife, Toshi and the Wall in a submersible. The mission appears to be going well, until a large object hits the submersible and causes it to lose contact with Mana One.
James "Mac" Mackreides, another crew member at the station, suggests sending Taylor down to attempt a rescue, after he suspects the object which hit the underwater ship to be Megalodon, a giant shark thought to have been extinct. Dr. Heller objects, but Zhang and Mac decide to travel to Thailand to recruit Taylor anyways. Persuaded to help, Taylor is able to reach the stranded three scientists. Toshi then sacrifices himself in order to allow Taylor to safely escape the submersible with the Wall and Lori.
Back at Mana One, the station receives a signal that sharks have broken through the depths of the ocean - including the Megalodon. The group decide to track and inject poison into the Megalodon. However, in carrying out this plan, the crew kill the wrong shark. The Megalodon then attacks the crew, resulting in the deaths of Zhang, Dr. Heller, and the Wall.
Morris later attempts to destroy the Megalodon himself by chucking grenades at it from a helicopter. However, he fails and is eaten by the Megalodon. Taylor and the remaining Mana One crew realize that the Megalodon is en route to a local Chinese beach, full of civilians. The crew manages to divert the attention of the Megalodon away from the beachgoers and toward them. Taylor is then able to scratch the Megalodon, causing it to bleed and attract other sharks to eat it, killing it.