On holiday in Saltbottle, Grandfather and Roo discover the Unsinkable, the boat which took them to the North Pole in the first book, lying in a sadly dilapidated state and 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 and 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 and Grandfather and 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 and rescued the Unsinkable.

Grandfather is off on an expedition to the North Pole to find the Last Polar Bears and with him goes Roo - a dog of character and strong views. The intrepid explorers set sail on the good ship Unsinkable and embark on the adventure of a lifetime.Eccentric, moving and very funny, their story is told through a series of extraordinary letters.