Gunther has been born into what he considers a boring life on the edge of the Catskill Mountains in upstate New York. Almost 16, about to become a junior in high school, he lives a life that is largely introspective. If only he could wake up one morning to see a replica of Coney Island's Cyclone running in the field behind his house, or if a hike to the spring in the back of his family's property could reveal the entrance to a cave, his life would be so much more exciting.
New Calar, the site of the regional high school, has suffered an unusual loss this summer. In early July, eleven students disappeared. All of them were alive and well on arising, but by noon they had vanished. Even now, a month later, no clue has appeared to indicate what happened to them.
When the entrance to a cave opens up near Gunther's spring, he and his younger sister June can't wait to investigate. Gunther has prepared for every occurrence—except the impossible cave-in. When the impossible happens and he and June are trapped inside, they have no choice but to traverse the cave to its far end—wherever that may be.
They have traveled two days before they are confronted by one of their missing schoolmates along with a creature that looks like a giant version of something grown on a microscope slide. Gunther knows about evolution of humans and animals on the earth's surface, but he is not prepared for what he now must learn about evolution that has occurred under the ground. He is especially not prepared to deal with a race of 1.5mm tardigrade captors—now grown 4 feet tall—or the expectations of his fellow captives, the ten surviving teens from New Calar High School.
Along with Hood, formerly one of his least favorite classmates, Gunther manages some form of communication with Teddy, the leader of the tardigrades. Trust between the species remains tenuous, especially after one of the students is found dead without any logical explanation. Convinced that escape is their only option, the teens must find their way out of the cave by an alternate route and survive an encounter with an army of tardigrades and an equally-overgrown Insect Guard.