Set immediately following the events of "We Didn't Mean to Go to Sea," this volume in the Swallows & Amazons series finds the Walkers once again sunk in gloom as their father, Cdr. Walker, has been denied his hoped-for leave, which will apparently prevent them from going on their planned family cruise to a region of marshes and islands suggested by their friend Jim Brading.
Then Daddy gets his Great Idea: he and Mother will "maroon" the children (including Ship's Baby Bridget and Ship's Kitten Sinbad) with camping gear and provisions while they go up to London, and the "shipwrecked sailors" will explore and chart the unknown regions in which they've found themselves.
The large island that will serve as their headquarters is surrounded by mud flats at low tide and has a farm (promptly christened a "native kraal" by Titty) near the middle of it, and with a small sailing boat of their own, the "Wizard," the self-sufficient and experienced Walkers anticipate no real trouble.
England