A small bottle-sized container floats in the ocean, led by the currents towards the coastline. Once ashore, a fisherman finds it and discovers a note inside. He starts to read...

It is 1916. A German U-Boat torpedoes a British ship in the English Channel. The ship explodes, leaving only a handful of survivors, including Bowen Tyler, an American on his way to France, and Lisa, a young English woman. With the assistance of the remaining crewmembers, Bowen decides to attack the U-Boat as it surfaces. They succeed and take over the German submarine. After a German soldier sabotages the radio, they attempt to signal a British ship but are shot at when it fails to recognise the origin of the signal. They decide to head west, towards America.

After six days of sailing, they still haven't reached the American coast. Perplexed, Bowen discovers that the compass has been tampered with, and that, instead, they have been heading south, towards South America. Taking advantage of the resulting confusion, the Germans reclaim possession of the boat and attempt to meet a German supply ship. Bowen and the British crew destroy the supply ship and retake the submarine.

The U-Boat finds itself in a frozen landscape. Bowen asks the captain of the sub to work with him to find a way out. They approach a snow-covered mountain whose abrupt cliffs fall directly into the ocean, providing no access for the ship. The captain believes it is the lost continent of Caprona, which navigators talked about two centuries ago. Running out of fuel, they attempt to find a way through. Noticing a warm current in the water, they dive and follow it underneath the mountain.

They surface in a lush tropical landscape, full of volcanoes, plants and trees not seen since the cretaceous period, when dinosaurs last roamed the earth. One such creature attacks the ship and is promptly killed and eaten. During the meal, Bowen and the German and British crews agree to set political differences aside and work together to stay alive. They set out to explore the island, leaving some crewmembers behind to build a camp. But local indigenous men follow them to a clearing, where they attack. At that moment, two Tyrannosaurus Rex also charge the group. The locals retreat, while Bowen and the others, armed with rifles, shoot at the dinosaurs and kill them. In the process, Bowen saves the life of Ahm, one of the locals, and brings him back to the submarine.

Through their difficult communication with Ahm, they discover that the island contains several pools of oil, which, once refined, could serve as fuel for the submarine. With Ahm as a guide, they head out to find the oil. Another tribe attacks them but is easily defeated. After finding the oil, they encounter more dinosaurs and are forced to retreat towards the camp. They use the U-Boat's weapons to kill one of the beasts. The camp is turned into a makeshift oil refinery and German and British crews work together to produce the fuel that will take them home. Ahm, driven by his need to rejoin his tribe, leaves the camp.

During a final expedition, Bowen and several crewmembers are attacked by Ahm's tribe. All are killed, except for Bowen and Lisa. Lisa is captured, while Ahm is killed attempting to save Bowen from the claws of a pterosaur.

The island is now torn by volcanic eruptions. In the confusion, Lisa escapes and, with Bowen, heads for the submarine. In the meantime, the remaining crew decides to abandon the camp and board the submarine. One German officer takes control of the ship, killing his own captain, and orders the ship to dive and abandon Bowen and Lisa. But the temperature inside the sub has reached dangerous levels and, as Bowen and Lisa approach the abandoned camp, the U-Boat explodes.

Bowen and Lisa are the only survivors. They learn how to live on the island. After 3 years, they decide to write their story. They put the letter in a small container, and throw it in the ocean, hoping that, one day, someone will find it and read it.