Anstace Roseveare and Humphrey Anthony are cousins and live near Applesendie in South Devon in a house named Fardingales. Anstace and Humphrey are sailing their boat "Susannah" to Looe (Cornwall) to meet up with Anstace's father and their friend Tom Vinton. They spend the day in Trevendick and Anstace is intrigued by a girl they meet in the bead shop as she does not seem to fit in there.
Overnight the girl and her brother (Kennie and Kevin Mackenzie) try to sneak onto the "Susannah" but Kevin cannot make it and goes under the water. Anstace and Humphrey help them onto the boat and look after them both.
Whilst Kevin is recovering, his sister Kennie tells them that they are running away from their only relative, Jago Halcrow. Kennie says that Jago Halcrow and his associates have tried to make Kevin burgle a house to steal important documents from there. Anstace and Humphrey agree to hide the Mackenzies on the "Susannah".
When they get to Looe they find that Anstace's father has succumbed to an attack of mumps and only Tom Vinton is there to meet them. They discover that Jago Halcrow is in Looe looking for the Mackenzies and decide to sail around to different places to throw Jago and his friends off the scent, since the cousins can't take the Mackenzies back to Fardingales as both Anstace and Humphrey's fathers are in quarantine.
After being chased over most of the south coast (including the Channel Islands) the "Susannah" ends up in Yarmouth where the lives of all of them are in peril.
If you want to find out more, click on Spoilers
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giving everything away!