The story is told through interviews with the survivors and the footage they shot during their investigation. Most of what we see is in the Blair Witch / Cloverfield style.
Journalist Natasha (Bel Delia) is investigating the abandonment of a plan by the Australian government to use old tunnels under Sydney as a water storage reservoir and recycling system. She suspects some sort of cover up, with reports of homeless people who live in the tunnels vanishing.
She lies to some co-workers to get them to join her in exploring the tunnels - sound man Tangles (Luke Arnold), and cameramen Peter (Andy Rodoreda) and Steve (Steve Davis). They have to sneak into the tunnels as the government refuses permission to film there.
Tangles is separated from the group and vanishes, leaving strange sounds on his equipment. Whilst searching they find a room covered in blood - and whilst they examine it, a night vision camera left on the floor is picked up by something unseen.
They search for a way out and are found by a security guard who leads them towards the surface, but he is attacked by something so fast it is seen only as a blur. Wandering lost they find him shortly afterwards, standing in a pool of water whimpering - with his eyes missing. As they watch horrified on the night vision camera an indistinct figure takes him from behind and twists his head completely around before dragging him off.
They find that light repels the creature, and stumble on a lair, complete with a pile of human eyeballs in the corner.
Natasha is captured and one of the cameramen attacked, but they manage to escape back to a populated underground station. The cameraman dies of blood loss, the other two survive.
Glimpses of the footage show that the creature appears to have three fingers rather than four, some sort of leathery skin, and an oddly shaped head with a narrow jaw. Ultimately we never learn just what it is or get a good look at it.