NOTE: This spoiler was submitted by Steve
We open to 5 years ago as professional diver Jonas Taylor (Jason Statham) tries to rescue a group of divers from a wrecked submarine. Jonas succeeds in getting some of them on board, including Dr. Heller (Robert Taylor), but something starts attacking the sub violently. This forces Jonas to make a difficult decision, against Heller’s protest, to abandon the submarine in order to escape certain death. They escape just in time as the unseen force destroys the submarine and disappears.
Cut to present day at Mana One, an underwater research facility off the coast of China. It is operated by a group of scientists spearheaded by Jack Morris (Rainn Wilson) and Dr. Minway Zhang (Winston Chao), and run by Zhang’s daughter Suyin (Li Bingbing), remote explorer DJ (Page Kennedy), Jaxx Herd (Ruby Rose) the aforementioned Dr. Haller, Mac (Cliff Curtis), Celeste (Jessica McNamee), Toshi (Masi Oka of “Heroes” fame), and “The Wall” (Olafur Darri Olaffson). They are conducting an experiment near the ocean floor involving bypassing a mysterious hydrogen, using a small submarine piloted by Celeste, Tosh, and “The Wall” and exploring what’s beneath. All seems well at first, but the trio is attacked and stranded by something large and fast, causing them to lose contact with Mana One. They later try getting power back up and running, but are attacked again and Celeste is wounded by a screwdriver getting rammed into her stomach. The rest of the crew, after much debate, decide to take Mac’s advice and call Jonas in to help rescue their stranded friends due to his experience with rescuing people from such a deep, dangerous area of the ocean. However, Heller is against this as he thinks Jonas went crazy from his traumatic experience five years ago and doesn’t believe Jonas’ claims that something attacked him and the divers he was trying to rescue. The plan goes ahead regardless.
Unfortunately, Jonas has clearly seen better days, as he’s seen in a Thai bar drinking regularly and pissing off local boat owners whom he keeps blowing off repairing their boats. Mac and Dr. Zhang meet with Jonas back in his room. Jonas at first tries to weasel out of the situation by offering the two drinks and (quite genre-savvily) preemptively rebuking any offers of money. In the end, however, he agrees to come along and help.
Meanwhile, Suyin tries to go down and rescue the trapped crew herself in a mini-sub shortly after Jonas arrives. Jonas takes off after her to help, but not before promising Suyin’s young daughter Meiying (Shuya Sophia Cai) that he’ll bring her mom back safe and sound. Suyin quickly finds the stuck sub and preps to tow them back to the surface when a giant squid attacks her. Before it looks like her sub will be damaged, the squid is quickly killed by the culprit of the previous attack: a 75-foot long prehistoric monster shark called a Megalodon. This shark is the same one Jonas encounter five years back that almost killed him, thus proving his sanity. Jonas briefly lures the shark away with some flairs and has Suyin escape back to the surface. He succeeds in getting Celeste and “The Wall” onboard his vehicle. However, the shark comes back for another attack, and Toshi sacrifices himself so the others can escape, stuffing a good-bye letter to his wife that he was writing earlier into “The Wall”’s pocket. The shark rams the downed second sub, killing Toshi, while everyone else gets to safety. Heller later apologizes to Jonas for not believing his claims.
The situation isn’t over yet, as the sub’s destruction has caused a temporary pocket of heat to vent upwards and allow the shark to move to normal ocean waters. While Jonas tries to leave and everyone else talks amongst themselves, Meiying is playing with an RC ball through the halls but is frightened by the sudden appearance of the shark. The shark tries to bite through the hull but fails and leaves after killing a whale. The group ultimately decides to track and poison the shark to death via a small boat. At first they succeed in tracking it using Jonas to shoot the tracker onto its fin and then reel him back in, just BARELY avoiding getting eaten by the shark. Then Suyin goes down in a plastic tank to poison it while the others lure the shark in with chum. This leads to an intense confrontation where the shark tries to swallow the cage whole with her in it. She succeeds in poison the shark, but her mask breaks, and she nearly drowns. Jonas dives in after her, and they almost get eaten by the shark. Fortunately, the poison kicks in and the shark dies. Jonas gets Suyin back aboard, and they resuscitate her back to life.
The crew initially celebrates by taking pictures of “The Wall” with the shark’s corpse before he falls into the water due to a prank from DJ. Jonas, however, notices the sharks teeth are smaller than the one from earlier. This is clearly meant to by a smaller, younger Megalodon they killed. Suddenly, the other bigger shark from before arrives, swallowing “The Wall” whole and grabbing the other Meg’s corpse off the boat with its teeth! This causes the boat to get capsized, and Dr. Zhang is severely injured in the process. The shark comes back around to try and eat Jaxx, but Heller sacrifices himself by distracting the shark to him instead, and he gets eaten. The surviving crew escape using motorboats and Morris calls in a support helicopter to shoot at the shark, successfully chasing it away. However, Dr. Zhang dies from his wounds in the boat, but not before tearful last words of pride to his daughter.
Later that night, Morris tells the crew that they’re shutting down Mana One and the authorities in multiple Asian countries have been called to deal with the shark. However, it turns out he’s lying through his teeth, and he actually called in some minor military support to try and kill the shark on his own using depth-charges dropped from the helicopter. He goes out on a boat with some others, and it seems to work at first, only for it to turn out they just blew up and killed a whale by accident instead, luring some nearby regular sharks (and later the Meg, who scares them off) to feast on its corpse. Morris gets knocked off the escaping boat and tries to climb up the dead whale, only to be eaten whole by the shark.
The crew finds out the shark is heading for a nearby popular beach in China (forgot the name) and plans to lure the shark away from the beach with whale noise recordings and then torpedo it to hell with the mini-subs. The shark initially attacks a nearby Chinese wedding party on another boat and seemingly eats the bride’s dog Pippin, before briefly terrorizing the locals at the beach. It stops attacking when it hears the whale noises and takes the bait. This leads into a massive chase between Jonas and the shark, as he leads it to Suyin who hits it with a torpedo, but it does nothing. Even worse, is that Jonas’ torpedo won’t fire. During the fight, a group of news helicopters trying to film the action stupidly crash into each other and the wreckage crashes onto the team’s boat, forcing everyone into the water. Suyin goes to help the others, while Jonas, in an attempt to protect everyone, goes head-on with the shark, slicking its underbelly open badly with the sub. He then gets out of the sub once the shark tries mauling it and stabs it through the eye, mortally wounding it. A dozen regular sharks move in to finish it off and eat it alive, ending the threat.
With the danger passed, everyone regroups aboard the wedding couple’s boat, including Pippin the dog, who turned out to have survived. Jonas and Suyin are presumably now a couple, their romance being hinted at throughout earlier in the movie. The movie ends with everyone celebrating their victory, and the oceans are now safe.