Stig is a caveman who lives at the bottom of the old quarry close to Barney's grandmother's house. Since the quarry is no longer in use, people throw all their rubbish away down there.
Barney finds Stig by falling through the roof of Stig's den. Barney and Stig become friends, even though they cannot talk to each other. Stig and Barney have a series of adventures together, including catching burglars breaking into Barney's grandmother's house. One night Barney and his sister Lou cannot sleep and get transported back to Stig's time where they spend a night helping his tribe complete a megalith. This may refer to Kit's Coty House, a barrow on the North Downs near Bluebell Hill in Kent, where King lived as a child.
The book is intentionally ambiguous as to whether Stig is real, or just a figment of Barney's imagination.