NOTE: This spoiler was submitted by Jeremy

The film opens during the Cretaceous Period. Several creatures prey upon the smaller animals, before a T-Rex goes after the little but deadly dino predators called Snappers. Then the Meg leaps out of the water and chomps down on the T-Rex before dragging its bloody corpse underwater.

In the present day, Jonas Taylor (Jason Statham) emerges from a shipping container on a boat sailing across the Philippine Sea. He is there to stop a group of men from dumping toxic waste into the ocean. After gathering his information, he proceeds to beat down the goons before he jumps into the ocean and is saved by Mac (Cliff Curtis) and Rigas (Melissanthi Mahut) in their chopper.

At an oceanic institute in China, Jiuming Zhang (Wu Jing) is testing out an exo-suit made for working underwater. He is the uncle of Meiying (Sophia Cai) and brother of Suyin, who has passed away recently, and Jonas has stepped up as a parental figure to Meiying. They attend an event for the institute, headed by Jiuming’s business partner Hilary Driscoll (Sienna Guillory). Jiuming pays tribute to Suyin and their father and vows to continue ocean-based research in their honor. Jiuming then presents Haiqi, a captured Megalodon that appears to respond to his commands. Meanwhile, Jonas notices a suspicious man named Montes (Serigo Peris-Mencheta) eyeing him at the event.

Jiuming tries to do a test where he uses a device to make noises that would keep Haiqi away from him. During the second run, Haiqi does not appear to listen, and the crew gets worried for Jiuming until he re-enters the station unharmed. Jonas says Jiuming is being reckless and shows concern for Haiqi’s behavior. When the crew leaves her unattended, Haiqi ends up breaking through the vents and swims into open waters.

Jonas joins Jiuming, Mac, Rigas, and DJ (Page Kennedy) head to the Mana One Research Facility to explore the Mariana Trench, meeting with another analyst named Jess (Skyler Samuels). Meiying stows away despite Jonas telling her to stay behind for safety. While going underwater in a sub, the crew notices Haiqi has gone loose and is with two other Megs. Meanwhile, Montes is leading a mineral farming operation before noticing the Mana One crew nearby. He blows the whole thing up, killing his own crew and causing the other sub to crash. Montes then reports to Driscoll, his boss, who tells him they need to prevent the Mana One crew from finding their work at the station.

After everyone picks themselves up, Mac (still at the station) determines that the sub was sabotaged. The crew exits the sub using the exo-suits with limited oxygen to make it to the nearby station. One crew member, Lance (Felix Mayr) gets killed after being pulled away by underwater creatures, while another one, Sal (Kiran Sonia Sawar) gets chomped up by a Meg after pushing a fellow crew member away. As they arrive at the station, one other crew member, Curtis (Whoopie Van Raam), has her suit implode before the water is drained since her helmet cracked.

The crew then begins to look through the station to find a way to call for help. They turn on a video monitor and contact the station, where they learn that Jess is part of Driscoll’s team and has betrayed the others. She releases the escape pods they needed and orders Rigas to shoot Jonas so that at least Meiying can return, but Rigas cannot bring herself to do so. Driscoll then speaks to the crew and reveals she is planning to steal Jiuming’s technology for their operation.

In order to get out, Jonas finds another sub and tries to drain the water out, but he passes out and is caught by Montes. He reveals he has a personal grudge with Jonas because he had him imprisoned a while back for his criminal activities. They get into a brief fight but Jonas is able to get the crew onto the sub to get out of there. Meanwhile, Mac and DJ watch video footage showing Jess sabotaging the sub and learning of her deception. Driscoll’s mercenaries go after them, but the two manage to jump into the ocean to get away. Unfortunately, they are caught anyway.

Jonas and the crew make it back to Mana One and fight the mercs to free Mac and DJ. The Megs then start making their way to the station as Montes rejoins Jess, who is his girlfriend. Jess speaks to Driscoll and assures her she is behind Meg-proof glass…and then one of the Megs chomps through it and swallows Jess whole, leaving Montes distraught. The station then starts to take in water, so the crew boards a raft to get away. The mercs follow them in their own raft, but they become Meg food as well.

The crew starts making their way toward a resort called Fun Island, while Jonas is creating harpoons with explosives and poison to use against the Megs. Montes then starts going after the crew with the remaining mercs. Nearby, a boat with other tourists gets pulled into the water by a giant squid.

Driscoll and her team attempt to fly off the island, only for the Snappers to emerge. One of them drags Driscoll away to her death, while some of the other mercs have unfortunate encounters with them as well. The crew then tries to get the tourists out of the water as the Megs begin to show up and start devouring some of the guests. Jonas tries to save some people, while DJ, Mac, Rigas, and Jiuming split off to help as well. Meiying also dives into the water to help rescue people (and a dog). The four encounter the remaining mercs, who hold them at gunpoint until Mac gets DJ to open a door, allowing the Snappers to come in and eat them so that they can run away. Mac and Jiuming find Driscoll’s chopper and start to make their way out, setting the Snappers on fire as they charge toward them after spraying them with jet fuel.

Jonas is able to kill one Meg by jamming the explosive harpoon into its face, but before he can go after the next one, Montes goes after him. They get into another brief fight before Jonas kicks Montes into the ocean just as the Meg leaps out and eats him. The squid then causes havoc on the resort and kills more people before Jiuming creates a bomb and jams it into the squid’s side. Haiqi then swims in and kills the squid. The last Meg goes after Jonas after he draws attention to it, and he uses a broken chopper blade to impale the Meg through the mouth.

Jonas, Mac, and Jiuming get in the water to swim back to land, but Haiqi starts coming their way. Jiuming uses his device to get her to swim around them and turn around. He thinks she really does listen to him, but Jonas suggests she is following dolphins.

The three get back to the beach with Rigas, Meiying, and DJ. Jiuming suggests that Haiqi got out of her enclosure because it was mating season and she might be pregnant. Jonas says it’s better not to think about it while they sit to have drinks and enjoy their survival.

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Jonas Taylor is now part of a crew to do further work in underwater research and stopping eco-terrorists. One of his business partners, Jiuming Zhang, who is the uncle of Meiying and brother of her late mother Suyin, has a Megalodon named Haiqi in captivity. Haiqi escapes and joins two other Megs in the ocean.

Jonas and his crew find that Hilary Driscoll, who invested in Jiuming's work, is using his tech to fund her underwater mineral mining operation. One of her mercenaries, Montes, has a personal grudge with Jonas, while the crew's analyst, Jess, is also working with the villains. Jess gets eaten while the Mana One crew gets to safety. As they start heading toward a resort, Fun Island, they learn that the Megs are headed in that direction, as well as a giant squid.

Jonas's friends take on the mercs, as well as prehistoric predators called Snappers, one of which kills Driscoll. Jonas fights Montes and kicks him into a Meg's mouth. He manages to kill one Meg with an exploding harpoon and impales the other through the mouth, while Haiqi kills the squid and goes back into the ocean, away from the humans. It is also suggested that Haiqi might be pregnant, but Jonas says they shouldn't worry about that since they just managed to survive.