Barney is a young boy staying with his grandparents on the chalk Downs of southern England. When he falls over the edge of an old chalk pit close to Barney's grandparents' house he tumbles down through the roof of Stig's den. When he looks round, there's Stig- apparently a 'caveman', with shaggy, black hair and bright black eyes. The chalk pit is disused and full of people's dumped rubbish.
Barney and Stig get on rather well together. They have to manage without language, as Stig speaks no English, but that doesn't seem to stop them. Stig's den is a place built out of discarded rubbish, that Barney enjoys.
Stig is Barney's secret friend, not because Barney doesn't tell anyone, but because no-one believes that Stig is real. They have a great time, improving Stig's den, collecting firewood, going hunting, and having all manner of adventures!