The Fat Controller has opened a new mine, and both the normal gauge and narrow gauge railways have tracks to it. An old boulder has had to be moved to make way for the mine, and Rusty is worried about it. Some time later the boulder is shaken loose and falls off the cliff it has been placed on and it chases Rusty down the line! Rusty manages to escape via a siding, but the boulder continues to thunder along! Back at the mine, Percy, with nowhere to run, sees the boulder up ahead, and it's heading straight for him...