When their mother goes on holiday, Barney and his sister Lou are sent to their grandparents for the summer. Any fear of being bored quickly evaporates when Barney is chased into the forest by local bullies, the infamous Snarget brothers.

Desperately trying to escape, Barney falls over the edge of a quarry pit, through a grass roof and straight into the home of a Stiganopithecus - a prehistoric caveman who lived over a hundred thousand years ago.

Soon Barney and his new friend 'Stig' are inseparable. Together they fight the Snargets and have some extraordinary adventures. But time is sunning out for Stig: the quarry is about to be developed and Barney;s grandparents are concerned that he spends too much time with his 'imaginary' friend. Can Barney find a way to get Stig back to how own time and people?

Based on the book by Clive King.