It's a year since their island adventure, and the children are staying at Spring Cottage on Castle Hill with Mrs Mannering while their real home is "decorated and cleaned." Mrs Mannering tells the children quite firmly to stay away from the castle at the top of the hill because the road up to it fell away in a landslide, and the route is treacherous. But a local "wild girl" by the name of Tassie, who is fascinated by both Philip and Kiki (in that order) and follows them around everywhere, shows them a safe path to the castle. Jack, spotting some eagles, wants to go up to see them, and Mrs Mannering agrees on the basis that Tassie can lead them safely. And the next thing that happens? That's right, into the castle they go...
An excellent book, but not as good as The Island of Adventure, the first book in the series. Better than some of the other books in the rest of the series, though.