Barney is a solitary little boy, given to wandering off by himself. One day he is lying on the edge of a disused chalk-pit when it gives way and he lands in a sort of cave. Here he meets 'somebody with a lot of shaggy hair and two bright black eyes' wearing a rabbit skin and speaking in grunts. He names him Stig.
Of course nobody believes Barney when he tells his family all about Stig, but for Barney cave-man Stig is totally real. They become great friends, learning each others ways and embarking on a series of unforgettable adventures.