The day Monica learns that her lovable, brilliant three-year-old son, Wills, has autism spectrum disorder, she takes him to buy an aquarium. It's the first in a string of impulsive trips to the pet store to buy animals as a distraction from the uncontrollable, crushing reality of Wills's diagnosis. But while Wills diligently tends to the growing menagerie, what he really wants is a puppy. And one Christmas, when Wills is six, Cowboy Carol Lawrence joins their family.

Like all dynamic duos, Cowboy and Wills complement each other perfectly. Wills is cautious, fastidious, and irresistibly tenderhearted. Cowboy, a rambunctious golden retriever, is overeager, affectionate, and impulsive. And from the moment Cowboy enters their lives, Monica sees her son step a little farther into the world.

Soon, the boy who could barely say hello to his classmates in kindergarten is sharing stories of his new "sister" Cowboy during morning circle. Children crowd around them at the park, and instead of running away, Wills, holding Cowboy's leash in his sweaty fist, proudly answers all of their questions. With Cowboy, he finds the courage to invite kids over for play dates, overcomes his debilitating fear of water to swim along beside her in the family pool, and, after years of gentle coaxing, Wills finally sleeps in his own bed with Cowboy's paws draped across his small chest.

Through it all, Cowboy is there, dragging him toward other children, giving him the confidence to try new things and the courage to face his worst fears. And when Cowboy turns out to need her new family as much as they need her, they discover just how much she has taught them - about devotion, about loyalty, and about never giving up.