Stig is a caveman. He lives at the bottom of the old quarry close to Barney's grandparents' house. Since the quarry isn't used any more people throw all their old junk away down there. So it is rather an interesting place to build a den.
How does Barney find Stig? Well, he just falls over the edge of the quarry and tumbles down through the roof of Stig's den. When he looks round, there's Stig, with his shaggy black hair and bright black eyes.
Barney and Stig get on rather well together. They have to manage without language, of course, but that doesn't seem to stop them. Stig's den is a brilliant place built out of discarded rubbish.
Stig is Barney's secret friend, not because Barney doesn't tell anyone, but because no-one really believes that Stig is real. They have a great time, improving Stig's den, collecting firewood, going hunting, and even catching some burglars who break into Barney's grandparents' house. It's really a collection of short-story adventures.
We know that Stig is a caveman, and really Barney hardly seems to give any thought to where Stig has come from until the end of the book. Then, during a very hot, sultry mid-summer's night, when Barney and his sister Lou can't sleep, they find themselves transported back in time and out onto the downs. To their surprise, they meet Stig, back with his own people, engaged in the construction of four gigantic standing stones. They spend a magical night camping out with the people of Stig's tribe, and helping to shift the final stone into position before sunrise.
Has Stig found a way to travel backwards and forwards in time, or is it as much a mystery to Stig as it is to Barney and Lou?