Thomas the tank engine and his railway friends learn all about how rumors spread and the importance of getting all the facts before leaping to judgment in Blue Mountain Mystery. When Sir Topham Hatt sends Thomas off to help the narrow-gauge engines at Blue Mountain Quarry after Paxton is injured by an unstable bridge, Thomas discovers a little green engine hiding in the tunnels. Some sleuthing reveals that green engine Luke is hiding because he once did something very bad; but what that something is remains a mystery. Thomas decides to find out exactly what happened, but his detective work yields some unintended consequences that just might get Luke thrown off of Sodor Island.