This book tells the story of Billy, a tank engine, the first of his class to be built, designed especially for hauling commuter passenger services in the suburbs of London. The book gives a detailed description of his design, with plenty of illustrations, such that the reader can learn about how steam engines work. As the story progresses, Billy is moved on from passenger services to rural services, then shunting, and eventually the scrap heap. But there is a happy ending - he is finally saved from the scrap man at the eleventh hour and restored to run on heritage services.