When mankind beams a radio signal into space, a reply comes from ‘Planet G’, in the form of several alien crafts that splash down in the waters off Hawaii. Lieutenant Alex Hopper is a weapons officer assigned to the USS John Paul Jones, part of an international naval coalition which becomes the world's last hope for survival as they engage the hostile alien force of unimaginable strength. While taking on the invaders, Hopper must also try to live up to the potential that his brother, and his fiancée's father—an Admiral—expect of him.
When the prehistoric warm-water beast the Crocosaurus crosses paths with that cold-water monster the Mega Shark, all hell breaks loose in the oceans as the world's top scientists explore every option to halt the aquatic frenzy. Swallowing everything in their paths -- including a submarine or two -- Croc and Mega lead an explorer and an oceanographer on a wild chase. Eventually, the desperate men turn to a volcano for aid.