- "And you will see, Diesel 10, some things are well worth waiting for."
- — The Fat Controller
Day of the Diesels is a feature-length Thomas & Friendsspecial released on DVD and Blu-ray in September 2011.
One day, Thomas is happily puffing along the tracks. That is, until he spots smoke in the sky ahead. He finds Percy already helping to fight a fire at a farmhouse, with the Fat Controller overseeing the effort to extinguish the blaze and starts to help fight the fire until Belle, the new fire locomotive arrives to save the day. After the fire is put out, Belle runs out of water and Thomas and Percy shunt her to the Steamworks. Once there, Belle is congratulated by the Fat Controller for her brave work. But he says that another fire engine is needed on the Island of Sodor. Belle recommends Flynn, so it is arranged to bring him over from the Mainland.
When Thomas starts to spend all his time with Belle, Percy feels left out and for some reason Diesel is being nicer to him than normal. Percy is supposed to deliver Flynn's hose to the Rescue Centre, but Diesel convinces him to go to the Vicarstown Dieselworks. When Percy reaches the fork, despite Thomas earlier telling him how diesels can be devious, he takes the track to the Dieselworks. At the Dieselworks, Percy is made to feel very special by none other than Diesel 10. Percy wants to leave so that he can deliver Flynn's hose, but Diesel 10 says that Dart can take it instead. Percy learns all about the state of the Dieselworks. The diesels say that they had told the Fat Controller that the Dieselworks needed repairing, but that he only listens to steam engines. Percy says that he will ask Thomas to tell the Fat Controller because he always listens to Thomas.
Meanwhile, Flynn arrives at the docks and sets off for the Steamworks. When Percy finds Thomas at the Steamworks, everyone is admiring Flynn. Percy tries to tell Thomas about the Dieselworks, but Thomas interrupts saying that the Fat Controller had asked him to show Flynn around the Island and he rolls over to the new fire engine/truck. Percy very sadly rolls away to Tidmouth Sheds.
The next day, Percy returns to the Dieselworks and learns that they do not have a working crane. Their usual crane, Happy Hook, is in very bad shape. Percy leaves to find Thomas, but when he arrives at the Steamworks, Thomas is not there. However, Kevin is and Percy tells him all about the Dieselworks and says that if Kevin went there, Diesel 10 would call him a hero. Then Thomas arrives and tells Percy that Flynn's hose is missing. Percy again tries to tell Thomas about the Dieselworks, but Thomas puffs away to talk to Victor. Percy searches all over the Island, but it soon grows dark and Percy returns to Tidmouth Sheds. There he finds that Flynn is in his berth. Very upset, he leaves and goes to the Steamworks. There he tells Kevin that he will take him to the Dieselworks. But before Kevin could say anything, they were off for the Dieselworks. Diesel 10 is delighted to have a new crane. Now that the Dieselworks has a crane, the place is soon cleared of piled up junk.
Diesel 10 asks Percy if he had told Thomas to tell the Fat Controller about the poor condition of the Dieselworks. Percy tells him that he had not, but that he would. Diesel 10 says that Percy should bring Thomas to the Dieselworks so that they would have his full attention. Percy reluctantly agrees and spends the night at the Dieselworks.
The next day, Percy arrives at Knapford only to learn that Thomas is looking for him. Thomas then arrives and asks where Percy was. Percy tells him, but before anyone can say anything, Gordon pulls up announcing Kevin's disappearance. Percy tells everyone that he is at the Dieselworks and Thomas asks if Victor knows. Percy admits that he does not and Thomas tells him that he must tell Victor right away.
Meanwhile, Kevin is playing with Happy Hook at the Dieselworks until Diesel 10 comes in and puts a stop to it. At the Steamworks, Percy tells everyone about the Dieselworks, but Thomas still does not listen. So Percy tells Thomas he must collect Kevin from the Dieselworks. Victor very crossly goes to tell the Fat Controller about the recent events.
Percy returns to tell Diesel 10 that Thomas will soon be there and that Victor had left the Steamworks. Diesel 10 says that Percy will lead the Diesels to the Steamworks so that they can take it over and that then the Fat Controller would listen to them. Thomas pulls in and Diesel 10 says that Den and Dart will take care of him. Den then tells Thomas to come to the back shed and he does so, reluctantly. Diesel 10 snaps his claw, causing sparks to land on some oily papers on the floor below, though neither Diesel 10, nor the other diesels notice it.
Percy is leading the diesels to the Steamworks and once there, Diesel 10 betrays Percy and announces that they will not give the Steamworks back. Percy, horrified, realises that Diesel 10 has been using him this whole time to take over the Steamworks and is not truly his friend and watches the diesels run amok in the Steamworks. Percy returns to the Dieselworks to find Thomas, but first sees that a fire had started in the main shed. Percy races to the Rescue Centre to get Flynn and Belle, but Belle reminds them that Flynn's hose is still missing. Percy thinks about how the diesels had tricked him and thinks that the hose must still be at the Dieselworks. So the three set off to rescue Thomas, Kevin and the Dieselworks.
They arrive at the back shed and find Den and Dart holding Thomas and Kevin hostage. Percy asks where Flynn's hose is and Dart tells him that it is in the rubbish. Belle and Flynn go to find it and deal with the fire, while Percy orders Den and Dart to let Thomas go, but they soon run away once Percy points out they will be incinerated if the Dieselworks burns down, enabling Percy to free Thomas and Kevin.
Now that Thomas and Kevin are free, Percy and Thomas roll to the front shed and watch Flynn and Belle put out the fire. Now that the Dieselworks are safe, albeit in worse shape than before, Thomas and Percy rally their friends to retake the Steamworks. The steam engines promise to help the diesels if they give back the Steamworks. Before anyone can do anything, the Fat Controller drives up along with Victor returning to see the damage to the Steamworks. Furious, the Fat Controller reprimands Diesel 10 and tells him that he has to fix everything that he damaged. Percy, Thomas and Kevin tell the Fat Controller that the diesels are sad because their Dieselworks is old and dirty. Then the Fat Controller reveals that it was his plan all along to restore the Dieselworks, but that things take time.
Soon the steam and diesel engines work together to restore the Dieselworks. At the opening, the Fat Controller says that the new Dieselworks is special because it shows what happens when Really Useful Engines work together and that he is very proud of all of his engines. Percy rolled up beside Thomas telling him that he feels proud and special when he is with him and Thomas said the same thing. The two best friends laugh together.
- 'Arry and Bert
- Diesel 10
- Sir Topham Hatt
- Lady Hatt
- Dowager Hatt
- Bridget Hatt
- Farmer McColl
- The Fireman
- The Tree Specialists
- Some Workmen
- Jem Cole (speaks only in Finnish dub; cameo in English dubs)
- Rosie (cameo)
- Hiro (cameo)
- Charlie (cameo)
- Harold (cameo)
- Captain (cameo)
- The Photographer (cameo)
- The Teacher (cameo)
- Spencer (mentioned)