Kate and Tom are conductors of the animal-saving Great Secret Intercontinental Railway. But her uncle Herbert has gone missing and there are more animals that need help. Out of desperation, Kate takes the Silver Arrow out on an unsanctioned mission, but she quickly discovers she's not the only one on the railway - there's a mysterious train called the Golden Swift out there, with an agenda of its own. Is it an ally? A rival? An enemy? Or all three?