- “I'll show that smelly diesel and those lazy steamers!”
Rusty, Rheneas, and Skarloey are sent to the incline railway to help Duncan with an important job. Rusty hopes that Duncan would stay out of trouble, but Duncan is impatient and behaves rudely towards the others. Skarloey and Rheneas do not like Duncan telling them what to do.
Duncan wants everyone to work faster, but Rusty and Rheneas tell him that they are proper engines and that they follow the rules. Duncan still tells them to work faster, so he shows the others how fast a useful engine can work by biffing one slate truck after another. Rusty warns Duncan that slate trucks would pay him back for being biffed. The engines see that Duncan will not listen, so they chuff away from the incline with their trucks of slate. Duncan pushes the trucks around and, while shunting some empties at the incline, gets his coupling tangled with a truck. The winch hauls the trucks and Duncan up the slope.
Rusty arrives in time to see everything, and Duncan's weight is too much for the winch and a coupling breaks, making Duncan run down the hill into the swamp. After Duncan is pulled out, the Fat Controller reprimands him for being an irresponsible engine as his impatience has caused confusion and delay and demands that Duncan apologises to the other engines. Duncan apologises to the engines, but the Fat Controller punishes him by working the incline (once repaired) until he learns to be patient and careful.
- Sir Topham Hatt
- Skarloey Bridge
- Rheneas Viaduct
- Skarloey Railway Slate Quarry
- The title of this episode is possibly a reference to Dunkin Donuts, an American global doughnut company and coffeehouse.
- This episode aired in AUS on Thanksgiving 2002.
- CGI smoke is used after Duncan falls into the swamp.
- From this episode onwards, Skarloey is lacking the gold stripe around the top of his funnel and his buffers are black.
- Thomas does not appear in this episode.
- Peter Sam is the only narrow gauge engine who does not appear in this episode.
- Duncan shouting "HELP!" in both dubs the same recording of Diesel's voice used from The World's Strongest Engine.
- This is the first time the Fat Controller says, "confusion and delay," which would become his catchphrase later on.
- The narrator says "Rusty, Rheneas and Skarloey chugged cheerfully through the Sodor countryside," but Rusty is a diesel and therefore cannot chug. In addition, Skarloey looked miserable and his eyes were wonky.
- When Duncan falls into the swamp, one of the trucks splatters mud under his mouth, but seconds afterwards, the mud is all over his buffers and mouth.
- A string can be seen moving off screen when the trucks fall in the mud.
- Books - Dunkin' Duncan and Duncan Goes for a Swim (Germany only)