Katy Robinson, an unhappy young foster child, daydreams constantly—of a proper home with her parents, of having good friends, and of being as popular at school as Sandra Magill. Sandra has an enviable lifestyle in the countryside, with her parents and her horse Tommy, and is always surrounded by friends. But when Katy dreams of doing something admirable to impress Sandra, the results aren't quite what she had in mind! Her initial shock at waking up to her new status as a horse gradually grows into acceptance. Being cared for by Sandra's posse, and becoming friends with Tommy, all help. But Katy-the-girl is officially missing and being looked for by the police—and when Sandra and her friends decide to undertake to find their schoolmate, Katy finds she is able to lead them in the right direction. Through various adventures, including putting the school bullies to flight, unexpected friendships and discoveries develop, and the question is—will Katy decide she prefers being a horse, or will she try to find a way to become a girl once more?