A team is exploring the most dangerous cave in the world, the Esa-ala System in Papua New Guinea. The caves look like a 2-kilometer deep vertical chimney in the middle of the jungle. A storm is approaching, so the local native workers under Jim Sergeant (John Garvin)'s command, are putting the equipment away. Victoria Elaine (Alice Parkinson) is the new girl in the cave. Crazy George (Dan Wylie) shows him a computer-generated digital view of the known parts of the cave: the Forward Base, - 2-km deep in a deep well within the cave system. the Ballroom, the Elevator, the Comms Box, Flowstone Falls, an empty area where there must be an exit towards the Solomon Sea.
Victoria Elaine makes a comment that this is where Carl Hurley (Ioan Gruffudd) is spending his money in. Carl, who is a millionaire, has stated that the Esa-ala Cave is not going to defeat him. Crazy George says that Frank McGuire (Richard Roxburgh) from The National Geographic are set on finding a way from the cave to the sea. Josh McGuire (Rhys Wakefield) is Frank's son.
He's a kind of prodigal son, as he had left without telling anyone. Josh complains that his father won't usually know evern if he's around. At the forward base, some people think that Judes (Allison Cratchley) is not ready to go down, as she's exhausted. Carl wants to get into action, but Carl has gone away to pick her girlfriend up. Frank tells Josh off, who won't have any of it. Everybody is telling Frank to take it easy with Josh, who is teaching Victoria how to make the descent - but she knows.
Carl will go down with them. J. D. (Christopher Baker) offers Judes a hot drink, which she appreciates. Victoria records Josh, who expresses boredom and disgust with Frank's obsession with caves; however, he has to go with him because of the divorce settlement, which forces Josh to spend a month with his father. During a diving exploration, Judes and Frank discover a beautiful natural chimney "like a cathedral".
Probably, that's the link with the exit to the sea. Frank says that there's a huge breathing bell above. Suddenly, Judes' tubes get loose, and she loses all oxygen. She has to share Frank's charge, as there isn't any oxygen replacement for her. J. D. tells Frank not to do it, as it may mean losing both of them. Judes drowns, panicking.
Frank blames Josh because of the replacements he should have brought but didn't. Josh blames his father, but the rest think that Frank was heroic. The tragedy distracts everybody, so that nobody answers base's radio warning; the storm has already begun. Josh thinks there must be a technical problem.
Josh tells Frank that he always puts too much pressure on people. Josh wants to leave, accompanied by Luko (Cramer Cain), who deems that Josh is too upset to climb all the way out by himself. Josh wants to throw the lantern/tooth which his father had once given him as a gift. At that moment, a ramble is heard, coming from outside the cave. Worried, they move on. Frank and Carl are left behind, wondering where the vertical tunnel is leading to. Meanwhile, in another communication point, Josh learns that the storm has become a cyclone.
The people outside is telling him to leave immediately, but Josh realises that there's no way of telling the people inside the cave because of the lack in radio communication. Luko wants to go on, as there's nothing they can do to help them themselves. J. D.
and Liz (Nicole Downs) climb towards the surface, but Josh refuses to abandon the others and stays behind with Luko. They try to ascend to avoid being drown. Some of the wall rocks are loose, causing two of them to fall all the way down. Josh survives, but Luko dies.
Lightpower is out. The way back is flooded, so there's only one path to go: forward, through the place they have discovered. Frank will have to teach Victoria how to dive. Frank has to kill one of the divers who was badly-hurt by drowning him - if not, he would be left behind in hours and hours of complete agony. Mercy dictates to drown him in this case. Frank tells Carl and Victoria that they lead secure lives, and that this is how you play when on an adventure. Victoria has to be the last, because she's the only one who can't dive; if she enters in panic, she'll cause all the people behind her to die.
So she will have to make do by herself with just the quick explanations she's been offered. It looks like Victoria is suffering from hypothermia. They give her warmth with their body, and when she's a little better, they moved on tied to each other with a rope. Having to dive and climb, soon George feels badly, from a severe case of depressurization. George and Josh fall a little behind. George tricks Josh onto letting him rest for a little while. He's coughing up blood but hiding it. When Josh talks to his father, he immediately realises that George wants to hide so as not to delay them.
They ran to look for him, but George submerges himself in the dark cold water in a tight corridor shutting his mouth so that he won't bleed. The trick works, and the rest have to move on without him. So they move on, trying to follow the flow of the river. They have to cross a circular chimney with turbulent water under them. Frank wants to go, but Josh is a better climber.
He prepares a rope around to cross to his own great danger. When the rope is secured, they start crossing one by one. However, Carl flashes his light into Victoria, making her slip down and her hair to get tangled in the rope. It's difficult to help her, but they're trying to devise a plan. However, it's painful as her scalp starts bleeding, so she takes the issue in her own hands and tries to cut her hair. Frank tells her not do to do, hang on and not do it, because if she cuts her hair without looking what she's doing - and her body position prevents her from seeing anything of that sort - she'll be cutting the rope to which she is attached as well.
The pain is hard to bear, so she succumbs and cuts her hair... and the rope, falling down to the meat-grinder water. Carl is frantic and tries to attack Frank, but Josh says that they're breathing because Frank has told them what to do. The way is testing and tiring.
It's difficult to know where to go. A new argument breaks. Carl and Josh hesitate in going on together. Frank decides to steal some oxygen and go on on its own. Frank and Josh sit down without moving forward, defeated. Suddenly, Josh realises that in a small track on the wall there's guano, and bats wouldn't have swum all the way to that crack on the wall. They have to climb slowly and painfully.
Josh asks his father to recite a poem which Josh's mother used to love. Finally, they reach a plain. All their lights are fading out one by one; when it becomes pitch black, they realise they can see each other. There is an opening in the roof of the cave: they find that a tank from some unknown Japanese army which fell down to the cave many years ago, and that the Japanese soldier died alone in the cave, even when there is a big opening with water and - when Frank and Josh arrive there - even vegetation and small animals. By the way, it looks that there are no remains of bats apart from their guano in that particular passageway.
They can't climb through that opening anyway, so father and son have to look for some other exit. They think they're close to the sea anyway. So they take another tunnel, and suddenly, oh surprise!, they come across Carl once again, with his face covered in blood. He talks and talks. He asks Josh for some food, and while he's eating like crazy, he asks Frank not to look to what he's done. In a pool of dirty water, a sewer-like conduct, there's the dead body of Judes with maggots all over it. The undertow pushed the body to the small cave and Carl has cannibalized it. Josh and Frank look at him in horror.
Carl also seems to be losing his grip on reality after what he's done. Anyway, the way out and the direction the current is going, they will have to dive with only a bottle of oxygen for the three of them. Frank and Josh are talking about what they should do next; when they reach an agreement, they are going to tell Carl, but he has some other plans. Carl and Frank fight. Carl runs away diving on his own through what they think it's the exit, leaving Frank deeply wounded.
Through his back, something has punctured Frank's lungs. He asks Josh to put him in the water, because he wants to die without too much pain. At first, Josh is appalled, but he finally complies. Josh cries, heart-broken. Using the last of the oxygen bottles, Josh follows Carl's way. There comes a moment when he passes by Carl's drown body. The bottle runs out of oxygen. Josh pushes forward, stopping here and there to breath a few drops of air.
However, there comes a moment when he's about to give up... Sunlight was over him: so close but so far. Suddenly, he remembers some of the words said by his father and he swims and swims upward and upward, towards the light. He emerges from the water, coughs up and breathes. He reaches the shore in a beach. Some children playing on the shore run towards him.