During an underwater cave exploration in Papua New Guinea, five people are trapped when a cyclone starts flooding the cave. With the water going up and the air running out, their only hope of survival is to travel through the unexplored underwater caves following the course of the river that leads into the ocean.
The film begins with the explorers 17-year-old Josh (Rhys Wakefield), expedition bank-roller Carl Hurley (Ioan Gruffudd) and his girlfriend Victoria (Alice Parkinson), reaching the cave by helicopter at the Esa'ala Cave site.
Upon reaching the cave it is made known that Frank McGuire (Richard Roxburgh), Josh's father and a master diver, has already set up a forward base camp deep inside the cave. The team has been exploring the cave system for months, and have set up an excellent communication network that allows communication with the camp from above. While Victoria and Josh rappel down the cave, Carl chooses to dive and parachute into it, reaching the base camp before Josh and Victoria.
While the guests get comfortable, Frank and diver Judes (Allison Cratchley) decide to explore an unexplored section of the cave known as Devil's Restriction. Squeezing through a narrow passage they reach a giant cavern with huge ...