- “Toothache? He's got teething troubles. That means he's new and is causing some problems. In his case it's in his cooling system.”
- ―Duck explaining teething troubles to the twins
Bill and Ben have been working hard in the clay mines and quarries near Brendam Docks. Their jobs are important and can make them naughty, especially when its very hot and dirty for the twins.
One day, Bill and Ben indeed become very naughty and argue over who has rights to some trucks in front of them. When BoCo shows up he tells the twins to stop quarrelling and tries to warn the two of getting in trouble with the Fat Controller, but the two ignore him and collide into each other at the points when they both try to take the trucks. The Fat Controller eventually intervenes and warns Bill and Ben to behave better, and then decides to send a diesel to help them, and hopes that he will make a good impression. BoCo also suggests that the twins just simply go back to work right away. Meanwhile, the Fat Controller has doubts about the idea, as he is concerned that the new diesel may have teething troubles more than confusion.
The Fat Controller is right; the new diesel, Derek, is having issues while going up Gordon's Hill, and, while halfway up his engine coughs causing black smoke to billow out of his exhaust. BoCo is then prompted to rescue him, who apologises for the breakdown and explains he has teething troubles. The news soon spreads, and Thomas explains the issue was caused by a malfunctioning cooling system. Percy misunderstands Thomas, and thinks he has "toothache." Percy then tells Bill and Ben about Derek and wishes them good luck, as they know they need another to help out. A manager then arrives and sends the twins to take their loads from the quarry to the docks, where they will be resting as there is hard work to be done the next morning.
That evening, Bill and Ben take their loads to sidings by the quay and head off to the sheds where Duck is resting. Duck, noticing the twins' worried looks, tells the twins that "teething troubles" means that Derek is new but is having frequent breakdowns, making the twins even more nervous.
The next day, Bill and Ben prepare their load of trucks until Derek arrives at the Clay Pits and helps the twins with a long train of China Clay trucks by pushing from behind with Bill and Ben pulling at the front. However, during the journey, they come to a hill and another breakdown from Derek occurs from overheating. Bill and Ben pull him and the trucks and it works.
That night, the Fat Controller arrives at the sheds and congratulates Bill and Ben for their hard work and explains that Derek is sent to the works. Bill and Ben properly make up in the end, agree to manage their work alone, and apologise to BoCo for being rude and, explain that Derek was very friendly, and Duck explains that friends always say goodnight to each others, and all four engines talk about teething troubles all night long.
- This episode is based on the inept attempts of the new Class 17 Paxman diesels to bank trains up to Ebbw Vale that occurred during David Maidment's time as Area Manager in South Wales: they used to run hot halfway up the bank and cut out, leaving the Class 37 to pull the train and banker.
- Stock footage from Cranky Bugs is used.
- Teething troubles usually occur while steam engines, diesels, and other machinery are operating for the first time after being constructed or restored to working order.
- S.C. Ruffey's face appears on a truck.
- This episode marks the last of two things:
- This episode marks Derek's first and only speaking role and his only appearance until Calling All Engines! (albeit in a music video).
- This episode marks Bill and Ben's only appearances in the fifth season.
- Despite his appearance in the episode, Percy does not appear in the nameboard sequence at the end of the episode in the Japanese dub.
- It is said that Bill and Ben heard an unfamiliar whistle (Derek's), but diesel locomotives have horns, not whistles. Also when the narrator says that line, there is a crew member's shadow on top of the coal chute in the top right corner.
- A grey cord can be seen in Bill's cab when he's in the shed at the end.
- In the first close-up of BoCo, his front wheels are derailed.
- When Bill and Ben crashed into each other, Bill's eyeballs were gone for a second as they were moved too high or too low.
- During the first night scene in the shed, a grey cord can be seen stretching along Bill's side.
- When Percy leaves Thomas, Thomas' eyes are wonky.
- The train next to Thomas passes him twice.
- A rare picture from The Ultimate Diesel Guide shows Derek smiling after he overheats.
- BoCo has Daisy's fourth season horn sound.
- Bill: "That's my line of trucks/cars!"
- Ben: "It's not. It's mine! Yours is over there!"
- Bill: "'It's mine!"
- Ben: "'It's not!"
- Bill: "'It's mine!"
- Ben: "'It's not!"
- (BoCo shows up)
- BoCo: "Stop quarrelling you two or the only thing you'll have left to share is..."
- (Bill and Ben race towards the trucks but collide with each other at the points)
- BoCo: "...trouble!"
- Bill: "Silly!"
- Ben: "Silly yourself!"
- Percy: "Hey you two! This new diesel's got toothache. Good luck!"
- Bill: "Why does Percy want to wish us good luck?"
- Ben: "Because he'll know we'll need it, a diesel with a toothache must be the worst diesel of all!"
- Derek: "I'm overheating again!"
- Bill: "Oh pah!"