The Famous Five are having a brilliant time—on holiday in horse-drawn caravans—and they've discovered a ruined castle nearby! The castle looked deserted from a distance, but is that a face at the window? Or is it a trick of the light? Just who is hiding in the castle?
The eleventh book in the series sees the Five camping out in two hired caravans. This is their second caravanning holiday, the first time being Five Go Off in a Caravan (no surprises there!). This time, however, they've borrowed the caravans from a friend at school, whose family has gone off to France this year. The caravans are the "old-fashioned types as used by gypsies"; they're bright and colorful and gay, one red and one blue, both picked out with black and yellow trim. They have a line of bold carving running around the edges of the jutting-out roofs, and large wheels. Inside the caravans are modernized and fitted out with bunks, a sink, cupboards and shelves, and cork carpet and warm rugs on the floor!