Rusty and the Boulder is the twenty-fifth episode ever made in Season 5.
A new quarry is being built near the mines and Rusty is afraid a gaint boulder will fall onto the line. He confesses his fears to Thomas and Percy. Then Edward arrives with a new machine called Thumper to help with the mining process. One rainy day, Rusty is gazing up at the boulder when a huge piece of rock falls onto the line. Rusty is worried by this, but his driver reassures him. The next day the sun is shining and Thumper is working on the rocks below the boulder. Rusty realizes that boulder is moving, but his driver does not believe him until boulder falls down in front of them and begins to chase Rusty.
Rusty quickly reverses to get away from boulder. Rusty and his driver stop at a junction and wait for boulder to pass by. They soon realise that boulder is behind them and race towards another junction. Rusty takes the line that goes uphill while boulder passes by them. Meanwhile, Skarloey is making his way up to the quarry when he sees boulder heading for him. He quickly reverses, but boulder is catching up. Skarloey slips into a siding while the boulder passes by. Boulder rounds a bend starts heading for Rheneas, who reverses and smashes into some buffers with boulder just missing him. At the yards, Percy is collecting trucks when he hears boulder. Percy closes his eyes and boulder crashes into the sheds. The Fat Controller decides to close down the mine and move boulder onto a hill.
- Rusty's small-scale model is used at the beginning and end of the episode, but his large-scale model is used throughout the rest. Because of this, Rusty's face seems to change size.