On holiday in Saltbottle Grandfather & Roo discover the Unsinkable the boat which took them to the North Pole in the first book lying in a sadly dilapidated state & up for sale At the auction the short-sighted auctioneer accidentally sellsthe boat to Roo
- sold to the old lady in the brown fur coat! Now they have to find the money & the Captain suggests a fishing trip to the Great Cod Banks in the Forgotten Sea Off they go but the trip ends in disaster when the boat sinks & Grandfather & Roo are cast away on a desert island They find their own Man Friday (called Tuesday) but he turns out to be a film director staying in a luxury hotel Meanwhile the Captain has found the treasure & rescued the Unsinkable