Just three per cent of the Planet's water is fresh water and it is our most precious resource. Where it flows or falls it controls the distribution of all terrestrial life. This episode follows the descent of rivers from their mountain sources to the sea and showcases the unique and dramatic wildlife found within its unexplored waters. The story begins with the ancient and mysterious tepuis of southern Venezuela, a series of isolated mountain plateaus and the setting for Arthur Conan Doyle's The Lost World.