During World War One a German U-boat sinks a British ship. The survivors are Bowen Tyler (Doug McClure) and Lisa Clayton, along with a group of the crew of the sunk ship. When the German submarine surfaces in the fog they manage to jump the crew and take it over. However, they soon get lost and end up near the lost continent of Caprona. They are running out of food and fuel so they make their way into the huge island through an underground river--and resurface in a lost world complete with dinosaurs and cavemen.
A few of the crew are killed off by dinosaurs and they befriend a curious caveman. But Caprona isn't just a lost world-- it has a very strange biology. Everything continually, physically evolves on the island so at one point that friendly caveman ditches them to join a more evolved tribe. The land itself is a huge volcanic crater--and the volcano starts to become active. The Germans take over the submarine and try to make an escape but they are boiled alive as the water overheats. This leaves Bowen and Lisa to survive on the island, going forever northward because they don't fit in with any of the other people there.
I've always liked this movie. Granted, I was ten-years old when I first saw it, but the dinosaur effects mostly hold up, particularly the plesiosaurs and pterodactyls and the actors are well cast.