Air date: October 16th, 1998 (UK), September 14th, 1999 (US)
High up in the mountains overlooking the Skarloey Railway, there is a large and mysterious boulder that, despite being almost perfectly round, has stood alone for a long time. One day, however, the decision is made to build a new quarry near the mines on the land below. Rusty is among the engines helping with the quarry's construction, and soon begins fearing the boulder may roll off its perch and fall onto the line. He confesses his fears to Thomas and Percy, saying he feels almost as though the boulder is watching him. Percy is skeptical since boulders do not have eyes, but Rusty asserts that there is something strange about this particular boulder.
Just then, Edward arrives, delivering a new machine to help with the mining process. The machine, named Thumper, is soon put to work mining stone. The men are pleased with Thumper's performance, but no one thinks to check the boulder despite working on the mountainside directly underneath it.
When a rainstorm comes through, the workmen pause their operations and leave. Rusty is left to gaze up and shiver at the boulder above. Suddenly, a large rock slab falls down and lands on the tracks in front of Rusty, much to his shock. His driver is concerned and thinks they should leave for their own safety, believing the rain has loosened the rocks on the cliffs overlooking them. Rusty, on the other hand, thinks the cause is the boulder trying to tell them to go away. He scurries away as a face seems to appear on the huge stone for a brief second, staring at Rusty hatefully.
The next day, with the rain gone, work resumes. Rusty is back at the same place from the day before, alongside Thumper, who is working even harder. However, soon he notices the boulder beginning to move. He alerts his driver, but he does not believe him until Boulder completely falls off its perch and lands on the tracks in front of them. Slowly but surely, Boulder begins to roll along the tracks. Rusty quickly drives himself backwards to get away as Boulder picks up more and more speed. They get far enough ahead that Rusty's driver stops at a junction to wait for Boulder to pass by. However, Boulder is nowhere to be seen, until Rusty's driver looks out the rear window and realizes it is barreling down on them from behind. Rushing along the line, they see a small junction to a track going uphill, so Rusty takes that route to allow Boulder to roll past and overtake them.
The engines are far from out of the woods, though, as Skarloey is coming up the line on his way to the quarry. Soon enough, he sees Boulder heading his way and immediately reverses. He soon slips into a siding, and the boulder passes by, destroying the water tower in between the tracks but ultimately leaving Skarloey unharmed. As Rusty comes down, Skarloey cries out that they have to warn the yards, but Rusty does not know how.
Boulder continues its uncontrolled charge down the mountain and soon reaches Rheneas, who races back as fast as he can to get away from it until he crashes into a siding. The boulder passes by him, much to the relief of his driver. Finally, Boulder reaches the yards, where Percy is collecting trucks. The green engine braces for impact, but Boulder thankfully rolls by him and takes out the neighboring sheds instead, engulfing them in flames.
When the Fat Controller inspects the damage, he decides to close the mine. Looking at Boulder amidst the destruction it has caused, he laments that they should have left this part of the island alone. Boulder is later moved to a small hill close by the yards so it cannot harm anyone or anything on the island anymore. Rusty is sure that, on a clear night, it is gazing high up at the mountains in the distance and that its sighs are being carried on to the wind where it once used to stand, proud and silent.
Tropes featured in the episode:
- Antagonist Title: The episode's
Working Title was "Boulder". - The Bad Guy Wins: Boulder's rampage leads to massive destruction on the Skarloey Railway. As a result, Sir Topham Hatt is forced to shut down the new quarry permanently.
- Darker and Edgier: One of the darkest episodes of the show.
- Disproportionate Retribution: The boulder, heavily implied to be sentient, is displeased that its domain has been disturbed. Its response is to try to kill everyone in the area, including those who are not even working at the quarry. It specifically targets Rusty, despite him doing nothing to provoke it.
- Indy Escape: The entire second half of the episode when the boulder falls off his cliff.
- Maybe Magic, Maybe Mundane: It's never said whether the boulder is somehow alive or not.
- My God, What Have I Done?: The Fat Controller realizes this after seeing the damage the boulder wrought after being disturbed by the new quarry/mine underneath the mountain where it had originally rested.The Fat Controller: We should have left this part of the island alone.
- Off-the-Shelf FX: Thumper's wheels and caterpillar tracks were taken from a 1:72 scale model of a Russian tank.
- Offscreen Teleportation: The boulder does this at one point when Rusty stops at a junction to let it pass by, only for the boulder to show up behind him.
- Oh, Crap!: "EEEYYOOOIIIKKEESSS!!!"
- One-Word Title: Once again, the episode's Working Title was "Boulder".
- Stock Sound Effect: In the UK dub, the episode contains a re-usage of James' "EEEEEEEEEEYYYYYYYYYYYOOOOOOOOOOIIIIIIIIKKKKKKEEEEESSSSSSSSS!" from "Busy Going Backwards", here said by Skarloey and Rheneas as the boulder goes after them.
- Surreal Horror: One of the things that makes the boulder memorably unsettling is because it is just so bizarre as an antagonistic force while also being undeniably dangerous, not just as a runaway massive rock, but a runaway massive rock that seems both able to teleport and think.
- Stuff Blowing Up: When the boulder crashes into the sheds at the quarry, a fiery explosion results.
- Vile Villain, Saccharine Show: The entire show is rather lighthearted as a whole, but Boulder proves to be the most dangerous villain in it. Besides Percy, it came close to squashing all of the engines it encountered.
