A civilian oil rig crew is recruited to conduct a search and rescue effort when a nuclear submarine mysteriously sinks. One diver soon finds himself on a spectacular odyssey 25,000 feet below the ocean's surface where he confronts a mysterious force that has the power to change the world or destroy it.

After a group of biologists discovers a huge network of unexplored caves in Romania and, believing it to be an undisturbed eco-system that has produced a new species, they hire the best American team of underwater cave explorers in the world. While exploring deeper into the underwater caves, a rockslide blocks their exit, and they soon discover a larger carnivorous creature has added them to its food chain.