While doing his latest piece, cynical American travel writer Pete McKell joins a group of tourists on a river cruise in the Australian Outback. He is immediately drawn to the feisty female tour captain Kate Ryan, though she seems to regard him as a bit of a yuppie. After a run-in with two of the locals, Neil and Colin, the cruise winds to a close, and Kate prepares to take the group back to shore.
When one of the passengers suddenly sees a flare shoot up into the sky, Kate believes it came from another tour boat up river, and persuades the group that they must go and offer assistance. A few miles up river, they come across the sinking remains of a dinghy, but no survivors. When something crashes into the bottom of the boat, causing water to gush in through a leak, Kate has no choice but to head for the nearest land - a small island in the middle of the river. As the group bickers about how they're going to get home, one of the tourists is suddenly pulled into the water by an unseen predator. With darkness approaching, everyone soon comes to the realisation that so is the tide. In a matter of hours, they'll be completely submerged.
Help seemingly comes in the form of Neil and Colin, who drive by again in their dinghy. But everyone's hopes are quickly dashed, when their boat is almost hit and sunk by whatever it is lurking in the murky waters. Neil manages to scramble ashore, but Colin is dragged underwater.
As night falls, Neil comes up with a plan to swim silently across the river to the shore, and tie a rope to one of the trees, creating a zip line for everyone to use to escape to safety.