Years ago, a plague called the mutagen began to spread, causing o-forms (normal animals) to give birth to mutababies (unrecognizable mutated animals). At the beginning of the plague, Trevin went around the country purchasing these animals in order to create Dr. Trevin's Zoological Extravaganza, a traveling zoo. His show used to bring great crowds, but the mutagen slowly spread across the country, making the mutoids much more common, and the o-forms practically extinct. Since then, Trevin's business has slowed immensely. Trevin continues to travel across the American South with his 12-year-old savant daughter Caprice. Caprice is herself a mutoid, whose DNA prevents her body from growing past the age of 2, even though she has the mind of an adult. Caprice is the brains behind Trevin's operation, but he encourages her to stay out of sight, since normal people fear and dislike human mutoids. The zoo caravan arrives in Mayersville during a softball tournament, in the hopes of finding bigger crowds. But the townspeople tear down Trevin's posters, and very few people attend the show. A couple tells Trevin that the town is dying because the rate of mutababy births among humans has grown so high. The elementary schools have shut down because there are no longer any normal human children. Caprice informs Trevin that there's no way they can stay in business. She suggests selling their equipment, letting the safe animals go, and killing the dangerous ones. That night, Trevin sees a bear-salamander mutoid attack his caged animals. He fights it off, ultimately killing the animal. When the police arrive, they recognize the mutoid as the child of a family in town, who had tried to smother it.
As onlookers arrive on the scene, they catch a glimpse of Caprice and are shocked to see a human child for the first time in ten years. The townspeople offer to pay Trevin, just to let them hold Caprice. Suddenly, Trevin gets the idea to turn his zoo into a new roving show: The Last O-Form Girl Child. The townspeople ask if Caprice can speak. Although Trevin fears that she will reveal her intelligence, Caprice plays the part of a normal human toddler.