The Diseasel is the twenty-first episode ever made in Season 2.
Bill and Ben are tank engine twins who work at a china clay quarry. Their jobs are to take the china clay in trucks along the main line for engines and ships.
One day, Bill and Ben arrange some trucks and leave to pick up more, but return to find them gone. Their drivers find a patch of oil and realise a Diesel is responsible for taking the trucks. Mishearing this as "diseasel", the twins argue who is responsible stating that "coughs and sneezles spread diseasels". The drivers stop the twins from arguing and make a plan to get the trucks back which involves removing the twins identifying plates and making them identical.
The twins find the trucks in a yard, with a large green Diesel at the front. Ben hides behind, with Bill going forward to demand the trucks back. The Diesel claims the trucks are his and tries to shoo Bill away, but when Bill disappears, Ben rides forward surprising the Diesel (thinking it's the same engine). This continues for a while, causing the Diesel to get dizzy. The twins then reveal themselves to him.
Edward arrives at the scene and asks what Bill and Ben are doing there. They explain that the "diseasel" stole their trucks, but Edward scolds them for their rudeness. The twins apologise and the Diesel forgives them. He introduces himself as BoCo and explains that he got his trucks mixed up with Bill and Ben's. Edward sends the twins to pick up BoCo's trucks.
After the two leave, Edward tells BoCo that Bill and Ben do not mean any harm, but can be maddening at times. BoCo laughs and replies that "maddening is the word".
- One of Bill and Ben's drivers says "We'll take away your names and numbers", but the twins weren't given numbers in the television series.