Do you want to know a secret? Salty the Diesel engine has a secret and all the other engines on Sodor want to know what it is. Join Thomas and Mavis as they meet Salty and discover the secret that Bill and Ben are still trying to figure out. Welcome Harvey and watch as he proves that, even though he may look different, he's a Really Useful Engine, and share in the reunion of Sir Topham Hatt and old friend, Elizabeth the Vintage Quarry Truck.