The film begins with a group of divers installing a power cable at the bottom of the Pacific Ocean, when a shark swims up and kills one of them. Six months later, lifeguard Ben Carpenter (from the first Shark Attack film) and his partner Esai drive out to sea to catch some lobster. While diving, Ben finds a broken power cable with a large shark tooth stuck in it. After he pulls out the tooth, he is caught by two other divers, and soon goes back to land. Later he posts a description of the tooth online, but as he cannot find a matching shark tooth on the internet, he names it "mystery shark". Natural history researcher named Cataline "Cat" Stone gets the message, finds him and looks at the tooth. She believes it to come from a huge prehistoric shark called Megalodon (Greek for "big tooth").
A small oil rig located on a small river in Louisiana hits it big and former oiler John Sanders (who quit when a blowout occurred on a rig he was the boss of and four men died) and his friend Emery are hired to take his ex-wife Kelly and her boss to the location. Nearby, some thugs do diving for stolen goods that were dumped nearby. Unfortunately, at the same time, a huge bull shark enters the river and starts terrorizing and killing people. Emery's people, a local tribe, believe that the shark is a spirit that supposedly protects the area where the well is manifested in physical form as vengeance for the oil rig.
In the opening, a collision between two alien spaceships occurs. The collision unleashes a capsule which heads toward Earth. The capsule collides with a small ship, causing an explosion. The capsule lands near an undersea research lab, the Oshona. The capsule, full of red crystals, attracts sharks. The sharks eat the crystals which cause them to become vicious killers, eating anyone and attacking anything that comes near them. Now, the researchers of the Oshona are stuck underneath the Bermuda Triangle. The Navy attempts to rescue them, but many people die in the process.
Le Méga Shark revient mais il va devoir faire face à un crocodile géant : le Crocosaurus. Attaquant beaucoup d'humains, les deux monstres se battent en duel. L'amiral Calvin et l'agent Hutchinson décident alors d'envoyer le lieutenant Terry McKormick et le chasseur Nigel Putman mettre un terme aux attaques des deux monstres.
Reynolds plays Caine, a gunrunner who becomes stranded in a small port in the Red Sea. He meets a seductive woman who propositions him to dive into shark-infested waters off the coast for scientific research. However, when Caine realizes the woman and her partner are actually treasure hunters, the action starts to heat up both above and below the water.
La société Blue Water, dirigée par Nathan Sands, crée pour l'US Navy une arme de guerre terrible : un hybride mi-requin mi-pieuvre, le sharktopus (alias S-11). Au cours d'une démonstration à Santa Monica, le sharktopus s'échappe et commence à dévorer les vacanciers sur la plage avant de parvenir à Puerto Vallarta, où il continue.
Dans un petit port de Floride, la vie des habitants est tranquille, jusqu'au jour où des barracudas attaquent les plongeurs. Confronté à l'agressivité des habitants, le biologiste Mike Canfield découvre d'une usine chimique déverse dans l'océan une substance qui développe considérablement l'appétit de quiconque l'ingère. Mais ce secret-là est jalousement gardé par quelques hauts fonctionnaires commanditaires d'un programme de recherches classé confidentiel et ne doit donc pas être rendu public. En suite, l'enquête se perd, abandonnant les recherches et le film plonge dans une magnifique histoire d'amour qui commencera à énerver un certain Akira Kimonodedjudo...
Tim est chasseur de serpents. Son job : capturer les serpents Corail, le plus redoutable des prédateurs du Pacifique Sud, afin d'en extraire le venin qui sera revendu à prix d'or à des laboratoires pharmaceutiques. Dak, responsable de l'équipage du bateau, connait bien ces serpents. Sensible au charme de Andi, la femme de Tim, il devient violent et menaçant. Il pourrait utiliser les reptiles contre son rival...
The film takes place along a stretch of coastline somewhere in Florida in the United States, where a local tourist spot has become plagued by a mysterious marine creature. Little do they know, the monster is the product of a secret military experiment — a genetic hybrid mutated from a common octopus and the prehistoric super-predator Dunkleosteus. Unfortunately, the creature has broken loose, and is now feeding on swimmers and tourists swimming or sailing along the coast. Also, the monster is only an infant, and will continue to grow if it is left to hunt much longer.
The coastal town of Hampton Bay is threatened when a 35-foot great white shark starts chomping up their vacationers. Days before the annual Regatta celebration, the townspeople are faced with financial ruin if something isn't done about their newfound shark problem. If that wasn't enough, the owners of the local amusement park have been subject to a hostile takeover from a prospective businessman looking to cash in on their prized property. It's then up to the sheriff, the park's owner, and a shark expert to head out and kill the shark before the summer economy is slashed and the park is demolished.