This book is set in Scotland and certainly has a Scottish feel to it, with its mountains, castles and golden eagles, though the country is not specified until The Mountain of Adventure, when Jack recalls observing an eagles’ nest at a castle in Scotland. It is the Easter holidays and Jack, Philip, Dinah and Lucy-Ann are on holiday with Mrs Mannering at Spring Cottage, which is set on a hill below a castle. Out on the hillside they meet Tassie, a local girl who lives in a tumble-down cottage with her mother and is allowed to run wild. Tassie cannot read or write but has a deep knowledge of animals and the countryside. She is excellent at climbing, sure-footed and has a good sense of direction, relying on her instincts to guide her: “She was more like a very intelligent animal than a little girl.” Indeed, with her bare feet, ragged frock and amazing agility she seems to be a part of the wild landscape around her — rather like a sprite or a wood-nymph. Perhaps because he too has a rapport with animals, she latches on to Philip and even brings him a fox-cub, which he names Button. The children’s happiness is complete when they discover that Bill Smugs is on a job in the area and plans to visit them soon.
Jack and the others are determined to explore the castle on the hill, despite the fact that the road to it has been destroyed by a landslide and is treacherous. Tassie shows them how to climb up the cliff behind the castle and put a plank across from a ledge to one of the slit-like windows so that they can get inside. The castle is “musty, dusty, fusty,” as Kiki loves to say, but Jack nevertheless decides to stay there for a few days, in order to photograph some eagles which have built their nest on a crag in the courtyard. Needless to say, puzzling things soon begin to happen. The pump in the kitchen is being used on a regular basis and there is a light in the tower at night. On further investigation, the children discover an underground room in which a group of men hold regular meetings.
In a complex plot involving the capture of the children, people hiding in suits of armour, secret passages and gleaming revolvers galore, the four children — and Bill — capture a gang of spies led by a man called Mannheim who, for obvious reasons, also goes by the name of Scar-Neck. Scar-Neck and his men are after the secrets of a new machine being worked on by Britain’s greatest military scientists, and have bought the castle because they know of a secret passage which leads from the castle and comes out just above the heavily-guarded clearing where tests are being done on the machine. It is a little sad, but fitting, that, at the end of the book, there is a tremendous thunderstorm during which lightning strikes the castle, leaving it in ruins. A dramatic end to a dramatic adventure!
One anomaly: after finally escaping from the castle, Bill has a talk with Colonel Yarmouth while Philip and Jack take a nap on the heather — heather which must be sopping wet after the storm!