The film begins with cave guide Josh (Rhys Wakefield) leading an expedition bank-roller Carl Hurley (Ioan Gruffudd) and his girlfriend Victoria (Alice Parkinson), to a cave reached by helicopter at the Esa-ala cave site. Once they have arrived they find that Frank McGuire (Richard Roxburgh) has set up a forward base camp deep inside the cave already. Frank also happens to be Josh’s father and a master diver. They have been searching and exploring the cave system for months and have a communication network that helps them talk with people above ground. Victoria and Josh rappel down to the base camp while Carl, a thrill seeker, dives and parachutes into the cave getting to the base camp before Josh and Victoria.
While Josh, Carl, and Victoria rest and set up for the rest of the expedition, Frank and fellow explorer, Judes (Allison Cratchley) go into an unexplored section of the underwater cave they call Devil’s Restriction. They go down through a narrow passage and find a giant, spacious cavern that goes far up with giant ceilings. As they return to base camp, Judes’ breathing hose breaks. They have no backup tank so Frank and Judes share a hose as they try to return to base as fast as possible. After switching the mask a few times, Judes panics and freaks out, refusing to give Frank the hose back. Frank, seeing that it is either going to be him or her, pushes her down so he can breathe and she dies right in front of him. The entire episode is captured by rovers following the divers and is witnessed by everyone at the forward base.
When Frank returns, Josh accuses Frank of murdering Jules and tries to attack him. Frank blames Josh for what happened because Josh was supposed to bring the backup tanks. He also says that Judes knew the risks before going down there. Meanwhile, a strong and dangerous storm rages on the surface but the people above ground are unable to get in contact with the people in the cave to warn them. The divers not knowing about the storm above make an attempt to return to the surface when water stars gushing down the cave. They can’t escape, and the rushing waters wipe out their base camp below. The divers regroup somewhere deep in the caves below somewhere relatively safe but now they have no way to get out or communicate with the camp above.
In the midst of the escape, Luko (Cramer Cain), one of Frank’s divers, has been badly injured. After much discussion, Frank takes it upon himself to kill him to put him out of his misery. Later on as they try to find a way out of the cave, George (Dan Wyllie), Frank’s best friend, comes down with decompression sickness and finds it difficult to go on. He decides on his own to stay behind so he won’t slow down the group as they make their way further down the dangerous cave looking for a way to escape. The group comes across a gorge in the cave and try to make their way across safely. Victoria, however, fails to follow Frank’s advice and gets trapped with the equipment and ends up falling to her death with the majority of the group’s equipment with her. They now have only one rebreather between all of them. The team decides to make camp and rest for a bit. Frank tells the group that the best way forward is for him to use the rebreather to dive down into the waters and find a way to come back and rescue them.
Carl, angry due to the loss of his girlfriend, steals the rebreather and dives in with the last tank. Frank chases after him but loses him in the maze of the cave. Frank and Josh are the only ones left. Josh happens upon some bat droppings and comes to the conclusion that there could be a passageway out nearby. Bats only use passages that humans can fit through. They make their way through the passageways and find a small opening that leads to the surface and a tank that crashed through it, blocking the way.
They decide not to wait to be rescued because they might starve and dehydrate to death before that happens, so they decide to find another way through the caves. Along the way, they find Carl who is still alive. He has become paranoid and crazy, convinced that they want to kill him. He attacks and mortally injures Frank. Carl dives back into the waters without any tanks to hopelessly find a way out.
Josh tries to save his father, but there is nothing he can do. He is suffering so Josh kills Frank out of mercy in the same manner that Frank did to Luko. Josh still has one single tank, so he decides to go back into the water and find a way through. All alone, his light dies underwater and he is unable to see anything. When this happens, Josh remembers a gift his father gave him, a boar tooth that has a little light inside. He uses it to guide his way through.
Josh’s tank finally runs out of air. Panicked that he is about to die, Josh calms himself down and remembers something his father taught him earlier in the film. He can use small air bubbles that are trapped on the cave ceiling. He does this intermittently as he tries to swim up towards the ceiling surface, finding small pockets of air to keep himself alive. Exhausted and on the verge of quitting, Josh sees a small ray of sunshine ahead and keeps heading towards it, finally breaking through the surface.
The film ends as he emerges out in the Solomon Sea and proceeds to swim towards land.
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