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.

Maverick Navy Lieutenant Commander Tom Dodge will never be a textbook officer, but he's a brilliant seaman who's always wanted to command a nuclear submarine — he's been given one last chance to clean up his record. Unfortunately, Admiral Graham, his nemesis, would rather sink the fleet than give Dodge his own boat. So, Graham stacks the deck against him and assigns Dodge to the Stingray, a diesel-powered WW2 submarine that can barely keep afloat. To make matters worse, Dodge's crew is a collection of maladjusted, mistake-prone misfits. Then, he's tagged the "enemy" in a crucial war game, and ordered to take on the U.S. Navy's best.

Merchant Marine sailors Joe Rossi (Humphrey Bogart) and Steve Jarvis (Raymond Massey) are charged with getting a supply vessel to Russian allies as part of a sea convoy. When the group of ships comes under attack from a German U-boat, Rossi and Jarvis navigate through dangerous waters to evade Nazi naval forces. Though their mission across the Atlantic is extremely treacherous, they are motivated by the opportunity to strike back at the Germans, who sank one of their earlier ships.

This film shows the duties of a young British sailor, Lieutenant Ellison, as his submarine H.M. Artemis sails the pack ice north of Iceland to gather data to complete a scientific survey for the Hydrographic Department.