|This article is about the magazine story. You may be looking for the episode.|
At the Dieselworks, Den and Dart are very busy. They always work really well together so it comes as a surprise to them when the Fat Controller arrives and sends Den to take Mavis' place at the quarry.
Sensing how nervous Den is, the trucks know that they can cause some mischief. Den does not know that one truck has it's brakes on, making it hard to pull. The next truck has no brakes on at all and flies off the rails when Den gives it a shunt.
Back at the Dieselworks, Mavis is waiting for Dart to bring her parts, but the little diesel is working very slowly. Mavis is sure he is missing his best friend. She then tells Dart that as soon as she is fixed, she can go back to work at the quarry and Den can come back to the Dieselworks. This cheers Dart up and helps him to work quicker.
At the quarry, Toby and Henrietta have some advice for Den. Henrietta tells the big diesel to be brave. So, Den plucks up courage as he starts to shunt and pull the trucks into place and none of them give him any problems at all.
Just as Den is finishing his work, one of the naughty trucks rolls away and heads straight towards Toby. Den gives chase and, with a burst of speed, gets in front of the truck just in time.
Finally, Mavis is fixed. She says goodbye to Dart and heads back to work at the quarry. When she arrives, she is amazed to see how tidy and ordered it is there. As he is leaving, Mavis thanks Den for his hard work. Den toots happily in reply.
When he gets back to the Dieselworks, Den tells Dart all about his day. He is happy to have done a good job, but even happier to be back with his best friend.
- Den is said to whistle at one point, but he does not have a whistle.