The five go on a holiday on the moors with a professor from Julian's school. On holiday, they make friends with the boy (Jock) on the farm nearby, he is quite friendly, but his stepfather is a whole other page. He scoldingly yells at the children to stay away from the railway yard with the spook trains. In curiosity, Julian and Dick decide to go down to investigate one night and see a spook train, but it makes no sense as that railway is allegedly unused.
In true Famous Five spirit, the five decide to look on the inside of the train tunnel and find a mysterious opening to a hidden entrance where Jock's stepfather has been selling stolen goods in the blackmarket. Of course, they get the police to arrest them and Jock and his mother are relieved to be rid of his stepfather.