Ten years ago in "The Great Crumble", society collapsed due to a viral pandemic of a disease known as the Sick (or the H5G9 virus), wiping out most of the world's human population. At the same time, many babies start to be born as mute hybrids with animal body parts and other characteristics.
Unsure whether hybrids are the cause of the virus or a result of it, many humans fear and hunt them. Gus, a part-deer hybrid, lives in the wilderness with his father whom he calls "Pubba". Pubba dies when Gus is nine years old and Gus discovers a box he buried beneath a tree that contains what Gus believes is a photograph of his mother, with the word "Colorado" written on it.
Gus accidentally starts a fire, which brings hunters who try to kill him, but he is saved by Tommy Jepperd, a lone traveler also drawn by the fire. Jepperd tries to leave Gus, but the boy insists that he escort him to Colorado to find his mother, and he relents.
The story also follows Dr. Aditya Singh as he tries to find a cure for the Sick, which his wife has lived with for years; and Bear, who is the leader and founder of the Animal Army which saves and protects hybrids. Bear is looking for her sister Wendy, a part-pig hybrid rescued by Aimee Eden, a former therapist whose sanctuary for hybrids is called "The Preserve".
