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 away 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.
How will Barney communicate with Stig and will anyone really believe that cave-man Stig is totally real?