Air date: August 8th, 2004
Thomas is sent to work at the quarry with Mavis as the quarry has an important order to fill. When Thomas arrives at the quarry, he is shocked to discover that Diesel will be working there as well. Diesel is not pleased to see Thomas, who tells Diesel that he has come to help Mavis. Diesel tells Thomas that steam engines cannot help as much as Diesel engines. Thomas scoffs at Diesel's insult and sets to work at once.
It is not long before Diesel is causing trouble; he pushes Thomas underneath the hopper, covering him in stones, complains that Thomas smells and calls him a "stinky old steam engine", and even says that the Fat Controller is planning to scrap steam engines. Thomas refuses to believe what Diesel says, but he cannot help worrying about it in the shed that night.
The next morning, Salty delivers fresh Diesel fuel from the Mainland. Diesel and Mavis are quickly filled up and Diesel boasts that the fresh diesel fuel makes his axles tingle. Then, just as Diesel rolls away to work, he coughs, splutters, and stops amid clouds of smoke. Diesel feels sick, as does Mavis, who is coughing out smoke from her exhaust. The Quarry Manager realises that the new diesel fuel has been contaminated with water. It is not long before 'Arry, Bert, and Salty break down too.
Having heard the news, the Fat Controller orders the Quarry Manager to send Thomas to collect some clean diesel fuel. Thomas sets off for the fuel depot and the workmen fill up two flatbeds with diesel fuel drums. Despite the heavy load, Thomas sets off and delivers the clean fuel to Salty, 'Arry, and Bert, and then, he makes his final delivery at the quarry.
The dirty fuel is drained out of Diesel and Mavis' tanks, and they are refilled with clean fuel. Mavis thanks Thomas and, after the important order has been filled, the Fat Controller arrives aboard Percy to congratulate Thomas for being Really Useful and a credit to the railway; even Diesel has to agree with that.
Tropes featured in the episode:
- Fantastic Racism: Diesel thinks Thomas can't work as hard as he can, as Thomas is, in his words, a "stinky old steam engine".
- Jerkass: Diesel, who goes out of his way to insult Thomas and pushes him under a hopper.
- Kick the Dog: Diesel pushes Thomas underneath the hopper, covering him with stones, and calls him a "stinky old steam engine".
- Laser-Guided Karma: For bumping Thomas underneath the hopper, Diesel gets his comeuppance after feeling sick due to the diesel fuel contaminated with water.
- Pet the Dog: Diesel's Smile of Approval to Thomas after helping him at the end, which also marks Diesel's first redeeming moment within the series.
- Smile of Approval: Diesel does this to Thomas in the end, impressed with his deed in giving him and the other diesels clean fuel.
- Teeth-Clenched Teamwork: Thomas is shocked when he has to work with Diesel in the quarry. Diesel himself isn't really pleased with this either.
- When He Smiles: Diesel gives Thomas a small smile at the end of the episode. While being more of an almost-smile, it still stands out as Diesel's first genuinely kind moment in the series.
