Show Off Diesel! is a custom Thomas & Friends episode.
Diesel boasts to James about being the important diesel on Sodor, but James tells him that the Express is the fastest train in the world. Diesel just laughs at him and tells him that he is a slowcoach, which makes James cross. Sir Topham Hatt tells Diesel that he is to take some passangers to a special lunch at Maron Station. The passangers climb on Diesel's coaches, and Diesel sets off.
On his way to Maron Station, Diesel goes up Gordon's hill and calls to James puffing by with the express (with Edward as his back engine), telling him that he is a slow steamie. This makes James upset. Edward tells Gordon to pay no attention to Diesel, as Diesel is nothing more than a bossy boxcoach.
After the passangers finish their lunch at Maron Station, Diesel decides to show them how fast he can go, so he starts to pick up speed. Although Diesel is proud of being the important and fastest engine in the world, he passes by several engines who think he is being very boastful and bossy.
The next day, when Diesel has to take the passangers to a boat, They warn Diesel to go a little slowly. Diesel thinks that as a splendid diesel engine, he shouldn't have to go slow. After he drops the passangers off on their boat, Spencer decides to pick up speed. He starts going faster and thinks the rest of the engines are slow. He passes by Duck, who warns him that the line ahead is dangerous, but Diesel pays no attention to Duck, and doesn't see the points are switched to a disused siding which hasn't been used for years. He swerves into it by accident, starts crashing through some buffers, and comes off the rails, also damaging his buffers in the process.
Sir Topham Hatt arrives onboard James, who brings Rocky to put Diesel on a flatbed so he can take him to the Sodor Steamworks to be repaired. Sir Topham Hatt is very cross with Diesel for being a show-off to the other engines and disobeying the passangers. He asks James if after he brings Diesel to the Steamworks, he would be interested in being the passangers' engine for the rest of the day. James is happy to, and gives the passangers a wonderful time.