Based on true events, Grace, her boyfriend Adam, and her younger sister, Lee, are on holiday in Northern Australia when they decide to take a tour down a river. As they drift into a swamp, their boat suddenly capsizes. Stranded in the flooded swamp, the three tourists must figure out what to do to survive as they realize they are being watched through the black water.

An adventure-loving couple convince their friends to explore a remote, uncharted cave system in the forests of Northern Australia. With a tropical storm approaching, they abseil into the mouth of the cave, but when the caves start to flood, tensions rise as oxygen levels fall and the friends find themselves trapped. Unknown to them, the storm has also brought in a pack of dangerous and hungry crocodiles.