Documentary-style thriller that unfolds largely in the dim, forgotten tunnels beneath Sydney, with the narrative framed by an on-camera crew chasing a story and intercutting with post-event interviews. Natasha Warner is a determined journalist who believes the government’s sudden reversal on recycling millions of litres of trapped water signals a hidden truth. As drought and water shortages push officials to take drastic steps, she, along with her team—Steve Miller the camera operator and Jim 'Tangles' Williams the audio engineer—plus a producer simply named Peter, aim to penetrate the tunnels themselves to uncover what really happened. Their investigation begins with a guarded entry by a security guard, Security Guard, whose refusal to permit access becomes a зов of the escalating peril they will face.
Inside, the group discovers long-abandoned homeless squats and spaces once used as air raid shelters during the 1940s, a stark reminder of the tunnels’ layered histories. They press deeper and stumble upon a vast underground lake that seems to hum with an almost living tension. In a room housing a World War II air raid bell, the tension breaks into something more unsettling: Natasha rings the bell to capture its sound on film, but the resulting blast distorts the recording. When Tangles moves the boom-mike to quiet the echo, Natasha rings the bell again, and suddenly Steve’s headphones pick up something profoundly disturbing. In a heartbeat, Tangles’ audio cable is yanked through a crack in the wall, and he vanishes from sight.
The film cuts to an interview back above ground, where Natasha watches the chilling recording for the first time. As the bell’s ring repeats in the footage, screams of agony creep into the sound, chilling the viewers with what lies hidden in the tunnels.
When the crew returns to the bell room, their equipment has vanished and a search for Tangles becomes a desperate hunt. They uncover a barred doorway and a room smeared with blood, where they find Tangles’ abandoned torch and signs that something grave has occurred. Natasha’s night-vision camera rests briefly on the floor, and when she looks again, the device sits in a different spot—hinting that something has moved and is watching them.
Panic spreads as they stumble into another tunnel where a security guard’s frantic pleas fade, followed by a sudden, vicious attack that drags him away in seconds. The trio hurries onward, seeking any exit, and eventually reaches the underground lake again. In the shallows, the guard appears, his eyes missing, and a tall, anthropomorphic figure steps from the shadows—the Stalker that stalks the group with ruthless speed. Natasha’s scream draws the monster’s attention, and the chase is on as they sprint through the maze of passages toward a glimmer of hope.
They reach a dead end with a barred opening into a street above, and then another corridor where light becomes a crucial ally. The monster is vulnerable to illumination, and the crew presses on with renewed caution. A side room reveals signs of living beings and, disturbingly, a small pile of human flesh, heightening the sense that the tunnels are home to something ancient, hungry, and dangerous. The primary camera falters, then powers back just as the creature lunges again, knocking Peter and Steven to the ground. Natasha slips away with the night-vision camera, moving in silence while the monster closes in.
Alone for a perilous moment, she confronts the creature and narrowly escapes, her lens cracked as it claws at her. Steven and Peter race toward the sounds of her screams, following the shifting glow of their gear as they pursue the threat. The night-vision footage finally shows Natasha dragged back toward the lake, where the monster attempts to drown her. The two men use the camera’s light to drive the stalking shadow away, allowing them to reach a well-lit tunnel near a train station. They haul a wounded Peter to a safer area on a nearby train platform, where Natasha, battered but resolute, pleads for help on a borrowed cell phone captured on CCTV.
The survivors spend the night in the tunnel, awaiting rescue that never fully comes. In the epilogue, a coroner’s report reveals Peter died from extensive internal bleeding, and Natasha steps back from journalism, choosing to resign rather than chase the truth any further. Steven continues his work as a news cameraman, while Jim 'Tangles' Williams remains missing, his fate uncertain to his family and to the audience. The police investigation closes inconclusively, labeled as having “contradictory evidence,” leaving the film’s central mystery unresolved and lingering in the memory of those who watched it unfold.
