Thomas and Percy and the Dragon is a simple story without much plot or point. In it, Percy the train sees what can only be described as a dragon; but when he tries to tell his friends about it the next day, they mock and disbelieve him. But when Thomas pulls up to the station with the dragon--a harmless parade float--following behind, Percy and the rest learn... something about bravery, I think? Because while the book is obviously aiming for some kind of moral about courage, it doesn't really come through clearly. Whatever they were going for, I'm not seeing it.
It's still a cute little story, though, and young fans of Thomas should enjoy it.