Thomas is over the moon when the Earl of Sodor chooses him and friends Percy and James to rebuild Ulfstead Castle. During their latest adventure, the Steam Team meets Millie, a bubbly French engine, and Stephen, an older steam locomotive with fascinating stories to tell. Meanwhile, Spencer and Gordon are stunned when they encounter two new engines, Connor and Caitlin, who are even faster than they are!
But disaster strikes when Stephen goes missing. Will Thomas find Stephen in time for the Grand Opening of Ulfstead Castle, and what is it that lies buried deep in the mine under the Castle…?