Shark Attack 3: Megalodon opens a shark attack on a diver working on an underwater cable off the coast of Mexico, and that death is just the beginning. The movie focuses on Ben Carpenter, played by John Barrowman. Carpenter is a resort life guard and is diving when he finds a large shark tooth lodged in the underwater cable. He is unable to identify what species the tooth came from so he asks for help on line.Carpenter's post for help ends up getting the attention of paleontologist Cataline "Cat" Stone, played by Jenny McShane, and she flies down to see the tooth. She lies about what kind of shark tooth Carpenter has found.
Meanwhile the shark continues attacking in the waters around a vacation resort. Cat and her team are able to get up close and personal with the shark and tag it with a shark camera. After the latest attack, Carpenter visits Cat to tell ask her about the shark attack and ends up getting her to tell him that the tooth is actually from a megalodon, a prehistoric mega shark. Cat takes him out on the boat the next day to find the shark, find out why it has appeared now, and try to protect the people who are in the water from this megalodon.