Thomas' Day Off is the eighteenth episode of the ninth season.
The Fat Controller announces to his engines that a new Diesel engine called Dennis has arrived on Sodor and tells Thomas that he is being given a day off after his recent spell of busy work. Thomas decides to spend his day off by going to the washdown and then to Bluff's Cove. On his way there, Thomas sees the new Diesel struggling to shunt some trucks of tiles. The new engine's name is Dennis and he tells Thomas that he cannot shunt the trucks, so Thomas decides to help Dennis, even though it is his day off. Afterwards, Thomas goes off to the washdown.
But Dennis is far from happy - he does not want to take the trucks of tiles to school because he is lazy. Thomas soon arrives back from the washdown and Dennis decides to take full advantage of Thomas' free time; he asks him to show him the way to the school. Thomas helps Dennis by telling him where to go when they reach a junction. But again, Dennis is not pleased as he watches Thomas heading towards Bluff's Cove. Then Dennis plays another trick. He blows his horn to attract Thomas' attention. Thomas comes back and Dennis pretends to that his engine had overheated. Thomas decides to help Dennis by finding another engine to help him.
Dennis is fed up. All of his tricks had backfired on him. Then Dennis decides to run away from his work so that another engine will have to take the trucks of tiles for him. However, as he runs away, Dennis derails and ends up wheel-deep in mud. Again Dennis calls for help, but no one is around to assist him this time.
Nearing Bluff's Cove, Thomas hears some urgent news from Percy at a signalbox. The workmen are waiting for Dennis' delivery at Bluff's Cove School and do not know where he is. Thomas knows of Dennis' whereabouts and goes off to find him. When Thomas finds Dennis stuck in the mud, he is very puzzled; he thought Dennis had broken down. Dennis owns up to his pranks, explaining that he is a very lazy Diesel. Thomas decides to help Dennis again. Together, they get the trucks of tiles to Bluff's Cove School. The Fat Controller is cross with his new Diesel's behaviour. Dennis promises the Fat Controller and Thomas that he will be a Really Useful Engine from now on.
- In Norway, the title is "Thomas Has A Day Off". The German title is "Dennis Is Lazy". In Japan this episode is titled "Lazy Dennis". It is called "A Holiday for Thomas" in Gaelic. The Spanish title is "Free Thomas Day".
- This episode marks Dennis' first and only speaking role until the nineteenth season episode, Diesel Trouble and his only appearance until The Great Discovery.
- The narrator says Dennis was huffing and puffing, but Dennis is a Diesel.
- Surely, Dennis' driver would make Dennis do his work.
- When Thomas leaves to find Dennis after hearing about the school roof from Percy, he goes forwards when he should have gone backwards. Likewise, in the next scene, Thomas approaches Dennis going forwards instead of going backwards.
- There is no explanation as to why Dennis derailed in the first place.
- Surely, Thomas would not have been allowed to run freely around the Island, even if it was his day off. The most logical thing he would be doing is staying in the shed.
- Sir Topham Hatt once said he sends lazy engines away, but he did not send Dennis away nor did he warn him.
- Dennis cannot move backwards so fast, because his prototype could go about 36 mph.
- The narrator says that Dennis is cross, but Dennis is sad.