Donald and Douglas are to take some equipment to Castle Loch for the re-opening of the castle, but Percy warns them about a monster that lurks in the loch; the twins are divided over whether Percy is telling the truth or not. On the way, the twins stop for a fallen tree and before they can move back, a landslide knocks the brakevan over. The Fat Controller promises to send help, but the hours pass and no one comes. Then, during the night, a mysterious figure approaches through the darkness. The twins are petrified, but it's only Harvey with a crane to help.