When a railway society comes to write a book about all of the engines they visit the Skarloey Railway, where they are especially fascinated with Duke's stories on the Mid-Sodor Railway. They look through all the records to see if there are any other surviving engines besides Duke, Peter Sam, Sir Handel and Bertram. While doing this, they come across a locomotive whose fate has not been accounted for. This engine's name is Smudger, and everyone is curious about what happened to him. Brackley and his team travel on the Arlesdale Railway to look for the remains of an engine, but to no avail. It's up to the narrow gauge engines to discover the truth behind Smudger and his disappearance.