The engines on the Island of Sodor are always very busy. They like being busy; it makes them feel Really Useful, but it seemed that no engine liked being busy more than Thomas.

One autumn morning, the Fat Controller came to Tidmouth Sheds with important news. "A new engine has arrived on the island," he boomed. "His name is Dennis; you must all help him get to know the Sodor Railway and Thomas," he added, "You have been working very hard lately and I am very pleased with you. As it is not very busy today, you can have a day off."

But Thomas wasn't so sure. "Does this mean I'll be staying in the Sheds all day?" he asked.

"Is there someplace else you would like to rest?"

Thomas thought. "First, I would like to have a long washdown. Then, I'd like to spend my day at Bluff's Cove."

"Very well," smiled the Fat Controller, "I'll make the arrangements at once." Thomas chuffed cheerfully; he was very pleased.

On his way to the Washdown, Thomas saw a diesel he had never seen before. The diesel panted and struggled as his wheels slipped.

"Hullo," Thomas whistled, "you must be Dennis. What's the matter?"

"These trucks are too troublesome. I can't shunt them."

"It's my day off," replied Thomas, "but I'll help you." This made Dennis very happy. "You have to be firm with the trucks," said Thomas, "and show them you're in charge, like this." Dennis watched Thomas biff and bump the trucks into line. "Come along, come along," he fussed, "there's work to be done."

"We don't want to; we don't want to!" grumbled the trucks.

Thomas was stern. "The sooner we finish, the sooner you can rest." The trucks stopped grumbling and Thomas and Dennis shunted them about the Yard together.

At last, all the trucks were in line. "All done," puffed Thomas; it was hard work, and he was tired.

"Thank you, Thomas," oiled Dennis gratefully and he watched Thomas chuff away.

Soon, Thomas was covered in bubbles. "Shiny and bright, shiny and bright. That's what I'll be to start my day right," he chirped cheerfully.

But Dennis wasn't cheerful; he was asked to bring a line of empty trucks to the warehouse. The workmen were waiting with crates of fresh new tiles.

"These tiles are needed to repair the Schoolhouse roof," said the Foreman. "You must deliver them as soon as possible."

Dennis was disappointed; he was hoping for a rest. He didn't want to deliver heavy trucks of tiles. Then, Dennis saw Thomas and that gave him an idea; he would ask Thomas to help him again. "Excuse me, Thomas," he said, "I have to deliver these tiles to the Schoolhouse, but I don't know how to get there."

"Don't worry, Dennis," Thomas puffed, "I'll show you the way." Dennis smiled; he thought his plan was working.

Thomas showed Dennis the way to the junction. He knew he would have to go the other way to get to Bluff's Cove.

"The Schoolhouse is further along that track," wheeshed Thomas. "You can't miss it."

Dennis watched Thomas puff away; he had wanted Thomas to take his trucks for him. Suddenly, Dennis had another idea. His Driver tried to make him go forwards…but Dennis wouldn't move. "Come along, Dennis," he said, "we have work to do." But it was no use. Then, Dennis blasted his horn again and again.

Thomas heard the noise; he was worried. Dennis watched Thomas come back. "What wrong, Dennis?" asked Thomas.

"I don't know. I can't move. My engine might be overheating. You'll have to take these trucks for me."

Thomas knew it was supposed to be his day off, but he still wanted to help. "They're expecting me at Bluff's Cove," he puffed, "but don't worry. I'll stop as soon as I can, and someone will find another engine to help you. Have your Driver put out some warning flags and be patient." And Thomas puffed away.

Dennis was cross; he wanted Thomas to take his trucks. Then, he had another idea. If I leave the trucks here, he thought, another engine will have to take them.

His Driver found the warning flags, but before he climbed out, he thought he would try to move Dennis once more. He tried to make Dennis go forwards…Nothing happened. He tried putting his engine in reverse…and Dennis suddenly raced backwards so fast, the coupling snapped.

"Stop, Dennis!" his Driver called, "you're going the wrong way!"

But Dennis didn't care that he was going the wrong way, he didn't care about the tiles, and he didn't care about Thomas. All he cared about was getting far away from his work. Dennis' wheels spun faster and faster.

Then there was trouble.

A Signalman saw Dennis racing along and changed the points. Dennis was surprised. He swerved into a siding…bashed through the buffers…and landed in the mud.

Now, Dennis couldn't move, and dark smoke billowed from his engine. "Help!" he called, "help me, please!" But this time, there was no one there to help.

"You silly engine!" shouted his Driver. "What were you thinking?" But Dennis felt so terrible, he couldn't say anything.

Further along the line, Thomas arrived at a red signal. Percy was there too. "The workmen are waiting for Dennis to bring the tiles to repair the School roof," he said worriedly. "The children can't go to School until it's mended, and no one knows where Dennis is."

"I do," wheeshed Thomas, "he's broken down."

"It's worse than that," said the Signalman. "I hear he raced away backwards and derailed. Thomas, you're the nearest engine. You must go back to help him right away." Thomas was very surprised and confused, but he was more worried about Dennis and the children, and he raced away as fast as he could.

Thomas quickly found Dennis. "What happened, Dennis?" he wheeshed. "I thought you broke down at the next junction and where are the trucks of tiles?"

"The trucks are back at the junction," said Dennis' Driver, "but I don't know what happened."

Dennis looked very shamefaced. "I'm sorry, Thomas," he said quietly, "I tricked you. There was nothing wrong with me at the junction. I just didn't want to move those heavy trucks. I've been a really lazy engine," he moaned.

Thomas and Dennis' Drivers were very cross, but Dennis' wheels wobbled, and his diesel fuel dripped. Now, Thomas could see that Dennis really was in trouble, so he decided to help one more time. "Hold on, Dennis," he puffed, "I'll pull you back onto the tracks. Then, after we deliver the tiles, I'll take you to the Repair Yard to be mended." And Dennis smiled his biggest smile ever.

Later, Dennis and Thomas finally pulled into the Schoolhouse Station. The Fat Controller was waiting. "Dennis, you have caused confusion and delay," he boomed. "You stranded your trucks, delayed the workmen and derailed yourself. What is the meaning of this?"

Dennis wheezed; he felt very bad and explained everything. "Thomas has shown me that being Really Useful is much better than being really lazy," he added.

The Fat Controller was pleased. "Well done, Thomas," he smiled.

"And thank you, Thomas," Dennis tooted. "From now on, I will try to be a Really Useful Engine." And Thomas was pleased too.