After word of a 4-headed shark that's swimming around the beaches of Puerto Rico begins spreading, aquarium owner Thaddeus Marshall enlists a marine biologist Angie Yost, and a group of interns consisting of Cait, Ram, Lindsay, and Sean to help him capture the shark so it can be an exhibit. However, when Sean gets into the water to find the shark, he is eaten, horrifying the others. Realizing they'll need reinforcements, the group hires local fisherman, and Angie's ex-boyfriend Red to help them capture the shark. As the group begins hunting the shark, it kills a humpback whale before chasing a group of tourists back to shore, and killing a police officer. Lindsay is eaten by the shark shortly afterward, prompting the others to return to shore.
Another shark attack soon occurs on one of the beaches, prompting Red, Angie, Cait, Thaddeus, and Ram to go back out to kill the shark instead of capturing it. Chief Sterling, and Deputy Angie Black volunteer to help as well, and the two boats head out into the harbor. In the meantime, the shark grows a fifth head on its tail. The shark soon arrives at Sterling's boat, and eats Angie before it chases Sterling back to shore. The shark heads back out into the water, and Red seemingly kills it with a bomb, although it is revealed to be another shark, and the actual 5-headed shark continues attacking the boat. Cait is knocked overboard, and Ram goes in to rescue her while Thaddeus distracts the shark by shooting it with a harpoon. However, Thaddeus' foot becomes entangled in the harpoon line, and he is dragged into the water, and eaten while Cait, and Ram reach the boat. A helicopter arrives to rescue the group, although the shark jumps into the air, and drags it into the water. When the shark returns, Red takes one of the bombs, and attaches it to the shark; the resulting explosion kills the shark, although Red survives, and returns to the boat, where he proposes to Angie, who accepts.