The story offers an early view of human evolution. The majority of the story is told through the eyes of the boy's hominid alter ego, one of the Cave People. In addition to the Cave People, there are the more advanced Fire People, and the more animal-like Tree People.
Other characters include the hominid's father, a love interest, and Red-Eye, a fierce "atavism" that perpetually terrorizes the Cave People. A sabre-cat also plays a role in the story.