From the publisher:
On summer holiday, the Swallows (John, Susan, Titty and Roger Walker) and the Amazons (Nancy and Peggy Blackett) meet up on Wild Cat Island. Unfortunately, though, the Amazons have a problem: their Great Aunt Maria has come to visit and she demands that the Amazon pirates act like "young ladies." Things get worse when the Swallows discover a very high hill that just begs to be climbed...
How the Amazons escape the Great Aunt, arrange a rendezvous, and mount an expedition to sleep under the stars on the summit makes a very exciting and satisfying story.
