Based on the story of the same name from Small Railway Engines.

Mike is annoyed of always being stuck behind animals, Rex boasts to the small engines about how easy wool deliveries are. The Small Controller gives him the first wool train. Willie the tractor driver accidentally drops his load of wool onto the tracks, which causes Rex to derail.

One night, Percy is frightened by a strange sound from under the bridge atthe Watermill. Toby is worried, though Thomas does not believe it to be true, until he too hears the sound. Toby and Percy believe it is a troll telling the story of how three goats had to cross a bridge with a troll under it. All three engines become terrified of passing the bridge until it is revealed the sound was only coming from a cow stuck under the bridge.