First, the audience follows an emergency call. The operator (Rebecca Clay's voice) tries to get the information about what is happening and to whom, but the conversation is like interrupted by static noise, silence and rambling.
In 2007, in the midst of the drought and water shortages, the NSW State government has unveiled plans to tap into and recycle millions of liters of water trapped in a network of abandoned train tunnels just beneath the heart of Sydney. However the government suddenly goes cold on the plan and it is not made public why. There is talk of homeless people who use the tunnel as shelter going missing, even though the government states that there are no homeless people in there. This, and the silence from the officials and ministers (Peter McAllum), leads a journalist, Natasha Warner (Bel Deliá) to begin an investigation into a government cover-up. She and her crew Peter "Pete" Ferguson, the producer (Andy Rodoreda), Steve Miller, the cameraman (Steve Davis) and Jim "Tangles" Williams, the sound technician (Luke Arnold) decide to investigate the story in the tunnel. They plan to enter the tunnels for themselves in the early evening. After being refused entry to the tunnels by a security guard (Ben Maclaine) and trying to bribe him which fails, they find an alternate entry and make their way into the tunnels undetected. The crew proceed to explore the tunnels for several hours and locate various abandoned homeless squats and sections used as air raid shelters in the 1940s.
They come upon the huge underground lake and it is while filming this with Natasha doing a verbal commentary to camera, that Tangles hears strange noises through the audio headphones. Thinking one of the others is playing a joke, he stops Natasha and begins several more times.
The group then heads to a room where the WWII air raid bell is. In order to get footage of the bell ringing, Natasha hits on it several times but Tangles says it is too loud and is distorting the audio. Tangles decides to take the boom-mike into an adjoining room as a way of reducing the sound levels. He leaves Peter with the headphones to listen for distortion in the bell volume. Natasha hits it again and Peter hears something very disturbing in the headphones, then immediately a whip-like sound, coupled with a sudden movement, and Tangles' audio cable gets suddenly pulled through the opening, snaps and disappears. The rest of the group panics and goes to look for Tangles who has disappeared. The scene switches to the interview, where Natasha is allowed to hear this recording for the first time. As the sound of the bell plays, the sound of screams of agony can be clearly heard before the headphone wire snaps. Upon going back to the bell room they find that all the equipment left in there has gone. Then while searching for Tangles, they hear something and find a room blocked off by scrap metal. Natasha places down the camera to help Peter and Steve. They get into the room and are horrified to find blood everywhere and Tangles' abandoned torch. At the entrance of the room Natasha picks up the camera saying it was not where she left it, so they review the footage and a glimpse of whatever it was that picked the camera up is seen. While they were investigating the room, something or someone had picked it up and is apparently stalking them.
They are forced to stick together if they want to get out of the tunnel to get help. They venture through the tunnel and hearing some noises of a person calling down, happen to come upon the security guard that had earlier refused them entry. They tell him that one of their crew has gone missing and that something has taken him but he tells them quickly to follow him and get out. Starting to follow him, a flash of light is seen and something pounces on the guard as he nears a corner, dragging him out of sight in an instant. The group flees, screaming in terror through blood spattered corridors. They hide in a room and stay there until they decide the creature has gone. Trying to find a way out of the tunnel, they return to the previously encountered lake. Hearing something in the dark, they extinguish all light and use the night-vision camera to look towards the sound. There, Natasha watches as the security guard, with water up to his chest and whimpering with pain from his empty eye sockets, is murdered by what appears to be an emaciated, tall humanoid who twists his head completely around, snapping his neck. She finally lets out a scream, attracting the creature's attention. The terrified group runs in startled panic.
They come to a dead end where they find a barred opening into a street above them, and Natasha breaks down, screaming for help. Peter eventually calms her down and they begin searching for an exit again. At one point, while climbing through a small opening, the creature attempts to grab Peter away. However as Steven turns around (thus shining the camera's light on them), the creature immediately retreats. Steven realises that the creature is vulnerable to light. Upon reaching another dead end, Natasha discovers a semi boarded up side route. Upon entering this new room, they find signs of some living being, and a small pile of fresh human eyeballs. The main camera loses power, causing Steve to frantically attempt to replace the battery. Just as the power switches the light on again, the creature attacks, knocking Peter and Steven to the ground. A panic-stricken Natasha runs off with the night-vision camera. On her own for a while, she stays in silence, but then hears a noise and is confronted by the creature who cracks the camera lens as it grabs at her, dragging her semi conscious with the small camera still recording. Now Steven and Peter, recovering from the previous assault, follow the sound of her screams and the creature's movements.
Through the nigh-tvision camera, we see Natasha dragged back to the lake. We hear the creature mutter in unintelligible, raspy sounds as it attempts to drown Natasha. Before it can succeed, Steven and Peter rush to help her, using the light to chase it away. As Steven helps Natasha get away, Peter can be heard screaming and challenging the creature, drawing them away from the other two. Steven manages to drag Natasha to a previous, well-lit tunnel near a train station. Natasha begs all the way to go back for Peter. Then Peter can be heard screaming for them. They rush back and drag a badly wounded Peter to the safety of the normal train platform. There captured on CCTV, we see Natasha amongst the morning crowd, calling for help on a borrowed phone. Peter is lying still and bloody and Steve is not helping but sitting dazed and exhausted against a pylon. It is about 7am, they have been in the tunnel all night.
In the epilogue it is stated that the coroner found that Peter died from extensive internal bleeding. Natasha resigned after the underground shoot, no longer working as a journalist. Steven still works as a news and a current affair cameraman. Tangles' whereabouts remained unknown and his family is still searching for answers. The police investigation was closed due to 'contradictory evidence'. No state government official or police representative agreed to be interviewed in the film.