Whilst exploring the woods Barney stumbles across the village bullies, the Snarget brothers, building a den. Chased by the brothers, Barney topples over the edge of a disused quarry. Barney sees a dark shape shuffling towards him and suddenly he is face to face with a caveman. Making friends with the caveman, Barney names him Stig.

Stig and Barney make a chimney for the cave out of old paint tins. But when Barney returns that evening, he discovers that the Snargets have vandalised the cave and even destroyed the new chimney.

When Stig discovers how to write on the cave wall using charcoal, Lou soon realises that Stig needs to learn how to hunt and catch food for himself.

Barney finds himself in danger when burglars hide out at the quarry after breaking into his grandparents' house and stealing their valuables. When a JCB arrives to dig up the quarry, Stig thinks fast to save Barney. Frightened of being caught Stig disappears.

The quarry is under threat as a development company has plans to fill it in. Stig is desperate to get his home back. Can Barney help him?

With the quarry now closed and Stig found shivering and with a temperature, Barney takes desperate measures to get Stig well and return him on the path back to his homeland.