A Shock for Mavis is a magazine story.
One evening at the quarry, Toby arrives with Henrietta to take the workmen home. Mavis decides to have some fun with them and tells them about the little men called trolls that work in the quarry at night. Toby does not believe Mavis, but the diesel reminds Toby how the quarry always looks tidy in the mornings and that is the work of the trolls. As Toby pulls the workmen home, he asks them if any of the men have seen the trolls who work in the quarry at night. The workmen all laugh and tell Toby that Mavis is playing a trick on him.
At the main station, Toby drops off the workmen and is about to retreat to his shed when the Fat Controller calls out and asks if they will take him to the quarry. Toby agrees, but is rather confused; the Fat Controller never goes to the quarry at night. Henrietta is more concerned about meeting the trolls.
When they arrive at the quarry, Mavis is still working and does not notice them arrive. The Fat Controller tells Toby that he has come to collect some scrap stones for his rockery and he has come at night because he does not have the time during the day. Moments later, Mavis pulls up and asks Toby what he is doing at the quarry. Toby decides to get his own back and tells Mavis that he has come to watch the trolls at work and they have spotted one. Mavis spots the Fat Controller in the distance with a wheelbarrow and thinks it is one of the trolls. Mavis hopes he is friendly as she has to spend the night at the quarry. As the "troll" gets closer, Mavis sees that it is really the Fat Controller, but Toby is happy to have fooled Mavis.