The movie centres on Thomas in the search for his new friend Stephen, who goes missing during the restoration of Ulfstead Castle.

Thomas, James, and Percy are assigned to pull a heavy goods train to the castle.

At the works, the crate is opened to reveal an old and rusty engine: Stephenson's Rocket, referred to as Stephen.

The Earl tells Thomas, Percy, and James that the castle will be opened to the public as a tourist attraction and that Stephen's special job is to be their tour guide.

Meanwhile, Thomas and Percy lose control of some Troublesome Trucks, which drag them downhill and push Stephen into the mine.