Mystery Fog! is a magazine story.
The engines are in Tidmouth Sheds waiting for their day to begin. Thomas can see the sunlight streaming through the window. Percy hopes it will not rain, but Thomas assures him that today will be warm and sunny according to his driver.
Shortly, the Fat Controller arrives to give the engines their duties. Thomas is the last to receive his job. The Fat Controller whispers to Thomas that he is to collect a package from Brendam Docks and take it to Cronk.
At Brendam Docks, Cranky is being very grumpy as usual. Thomas tells him to cheer up; it's a beautiful day. Cranky says it won't be for much longer and tells Thomas to look at what's written on the side of the box he's unloading. On the side of the box are the words "Fog Machine". Thomas asks Cranky what it is for, but Cranky doesn't know and just tells Thomas to take it away; they have enough fog drifting onto the island as it is.
On the way to Cronk, Thomas passes Henry who is in a very good mood. He says it is so clear out today that he can see right across to the Mainland. Thomas decides not to tell him about the fog machine.
When Thomas pulls into Cronk station, there is a man called Mr. Director waiting for him. Mr. Director is delighted to see the fog machine and explains that he is filming a mystery movie and needs lots of fog. A porter arrives with a trolley and tries to lift the heavy fog machine off Thomas' flatbed and onto the platform. Unfortunately, the fog machine topples over with a loud crack. Mr. Director opens the box to check the fog machine for damage, but is very upset to find the machine in pieces. Thomas has an idea and says that he can make lots of steam which looks just like fog.
Mr. Director is very pleased with Thomas' idea and asks him to move further down the platform. Powerful lights have been set up and crowds of people are dressed in old-fashioned clothes. Mr. Director calls "action" and Thomas makes clouds of steam pour out of his funnel. The steam billows all around the actors and makes everything look very eerie, just like fog. Thomas is very pleased; he never imagined he'd be a fog machine and a movie star at the same time.
- The clapper board in the story shows the date that the story was first published.