Dennis is a small diesel who lives on the Island of Sodor. He always tries to be friendly to the other engines, but he can be rather lazy. Dennis is known for playing many tricks to get out of work, pretending to be ill, doing the smallest jobs, going slow to make his work easier or saying he needs help and making other engines do his work for him.

When Dennis first came to Sodor, he saw how good Thomas was at handling trucks. Dennis tried to trick Thomas into doing his job for him, but it didn't work. He tried to run away, but only got himself into trouble. After that, Dennis wanted to be Really Useful, but he found it hard to change his ways.

One day, Dennis' twin, Norman, found him resting in the goods shed. "Wake up, sleepy shunter," he honked, "there's work to do!"

"I already finished my shunting," groaned Dennis.

"Well, now, you have to shunt my trucks away. I have to collect some ballast from Arlesburgh." And Norman raced away.

Dennis roared into life and purred slowly out of the shed. "It's not fair," he grumbled. "I already did my share of the work."

"You've rested long enough, Dennis," said his Driver. "Come on, you want to be a Really Useful Engine, don't you?"

Dennis sighed. "I suppose so."

Later, Dennis was asked to take some trucks to Wellsworth. When he arrived, he backed into Edward's Shed to rest when the Stationmaster walked up. "Thank you, Dennis," he said. "Now, Derek will be bringing some trucks from Brendam Docks for Edward to collect later. Since you're here, would you mind getting Edward's trucks ready for him? Then, he can leave early." And he left.

Dennis was disappointed. No one said anything about shunting trucks here, he thought. Dennis refused to leave the Shed and nothing his Driver did would make him move.

When Derek delivered his trucks, Dennis pretended to be asleep and when Edward arrived, he was surprised to see his trucks weren't ready in spite of Dennis being there. Edward didn't say anything, but he wasn't happy.

That evening, when Dennis got back to the Dieselworks, Norman was waiting for him. "You've done it again, Dennis," he fumed.

"What did I do, Norman?" asked Dennis.

"Push your work onto another engine. Edward told me he saw you sleeping in his Shed and that his trucks weren't ready."

"No one told me about that job this morning," said Dennis, "and Edward would've shunted those trucks himself if I wasn't there. Anyway, he wasn't late with his train."

Norman frowned. "That's not the point. You could've helped Edward, but you didn't. You, Dennis, are the laziest engine I've ever known." This made Dennis very cross.

The next morning, Dennis refused to leave the Shed. "Come on, Dennis," urged his Driver, "we'll be late for our first job."

"I won't go," Dennis huffed. "Have someone else do it."

At last, his Driver gave up and went to tell the Dieselworks Manager. Dennis watched the other engines leave. That's it then, he thought grumpily, I'll just stay in my Shed forever.

When the Fat Controller heard the news, he was upset too. "Oh dear," he sighed as he looked over his timetable, "that Dennis is always causing confusion. If only we could put a stop to it." Then…he heard a familiar horn outside his Office. "Of course!" And he went out to the platform immediately.

Later, Dennis was enjoying his rest, but he was still grumpy from what Norman had said. Presently, he heard a horn…and a very unusual visitor pulled up. "Daisy! what are you doing here?"

"I wanted to talk to you, Dennis," replied Daisy. "Why are you still in your Shed?"

"I'm not working anymore," Dennis replied. "There's always too much for me and it's all too hard."

"Really? I heard you're a very reliable engine, Dennis. Seeing how hard Norman works, I'm not surprised."

Dennis looked at his buffers. "He should know how I feel. He used to break down so often that he used to spend more time in the Repair Shed than on the railway, but ever since he got that new engine of his, he never seems to stop. I can't understand it; I would've thought he'd be rushed off his wheels, but he seems to work harder than ever."

"…You know," said Daisy slowly, "I used to be a lot like you."

"I don't see how; you're a diesel railcar. You don't even have to pull anything."

Daisy sighed. "That may be, but I was still lazy. After Thomas crashed into the Stationmaster's house, the Fat Controller brought me here to help out. Thomas usually delivered milk on his first run, but I refused to work if I had to. I made excuses until they let me leave it behind and Percy and Toby were very cross with me.

"Then, I made myself look silly again when I refused to shoo a bull off the line and some children teased me about it. After that, I felt very silly and embarrassed.

"Some days later, Percy had an accident with some trucks at crashed outside Elsbridge Station. I was asked to help Toby clear the wreckage. I didn't want to at first, but I was told I was the only other engine available and if I didn't help, it would cause even more confusion, especially for the passengers.

"The next day, the Fat Controller arrived. He said he wasn't pleased with me telling lies and would send lazy engines away, but Toby told him how helpful I had been, and he gave me another chance.

"Ever since, I've tried to be a Really Useful Engine. I admit that I still make some mistakes, but everyone does now and again."

"What does any of that have to do with me?" asked Dennis.

"I'm sorry to say this, Dennis," Daisy replied, "but you're worse than I ever was. I still did my work while you prefer to stay in the Shed all day. What if the Fat Controller sends you away next? What if you never see Norman again? What's more, what if the Fat Controller took your wheels away and left you here forever?"

Now, Dennis felt bad. "I never thought of that," he said. "Norman's my brother; I don't know what I would do without him and if I never left the Sheds again, that would be boring."

"Well," smiled Daisy, "none of that has happened yet…and if I can change my ways, so can you."

Dennis thought for a moment…Then, he smiled too. "You're right, Daisy, and I'll do whatever I can to be Really Useful."

"I'm glad to hear you say that, Dennis," said a voice…and the Fat Controller stepped out from Daisy. "I know you can be Really Useful. All you have to do is work hard and do everything you are asked."

"Yes, Sir, I will, Sir."

"Good, now, there are some heavy trucks that must be collected in Vicarstown. Can I depend on you to deliver them to Brendam Docks?

"Right away, Sir," tooted Dennis and when his Driver returned, his engine roared into life, and he raced away as fast as he could.