When it was Christmastime, Diesel 10 became jealous of the steam engines for having more decorations at Tidmouth Sheds than he and the diesels had at the Dieselworks. He began stealing decorations from the sheds to bring to the Dieselworks, enlisting a reluctant to help him. Diesel 10 was caught by Percy, who chased him back to the Dieselworks with the rest of the Steam Team, where Diesel 10 found out that Sir Topham Hatt had, in fact, had Paxton deliver some decorations to the Dieselworks. The next day, Diesel 10 gave the decorations back to the Steam Team, but as most of them had been damaged, he gave them the Dieselworks' decorations too, as he and the other diesels considered to be a good enough decoration.
Diesel 10 is an vile, conniving, bullying, and raving diesel locomotive. He absolutely despises steam engines, and would call them taunting names like "" and "." He is Sodor's strongest but not quite largest diesel, his power being enforced by his slightly malfunctioning claw, Pinchy. He can use this deftly, and is even able to use it to make a sculpture. Pinchy, though, seems to have a mind of its own, as it has been shown to "punch" Diesel 10 in the face from time to time, which he hates very much. All engines, steam and diesel alike, fear and respect him.
When Percy was experiencing a falling-out with Thomas, Diesel 10 decided to manipulate him into helping him restore the , by having him visit the Dieselworks and befriend him. Diesel 10 managed to convince Thomas to arrive at the Dieselworks, where he was held hostage while he, Percy and the other diesels went to the . Once there, Diesel 10 betrayed Percy and took over the Steamworks. Percy returned to the Dieselworks to rescue Thomas, when he noticed that sparks from Pinchy had set the main building ablaze. After Thomas was saved and the fire was put out, Percy, and the rest of the went to the Steamworks. After a brief argument with the diesels, Sir Topham Hatt arrived and, after being told about the state of the Dieselworks, agreed to have the place restored. Diesel 10 and the other diesels worked alongside the steam engines to restore the Dieselworks, and he became the new head of the Dieselworks, with and taking over in his absence.
After Thomas was found prior to the reopening of , Diesel 10 was among the diesels present at the quarry when they heard the news. Later, when the was opened, Diesel 10 was present at the rescue centre's grand opening, standing on a cliffside overlooking the rescue centre and the steam engines below, threatening to return.
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