Everybody had told Barney not to play to near the edge of the chalk pit. But today was one of those grey days when there was nothing to do and nowhere to go. Except to the chalk pit. The dump.

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.