Mrs Mannering — Philip and Dinah's mother and Jack and Lucy-Ann's adoptive mother — is determined to take her four children on a holiday where they won't get caught up in any more of the extraordinary adventures that seem to find them wherever they go. A cruise among the Greek islands seems a safe enough idea. Then, during their trip on board the Viking Star, the children find another ship — a old model ship in a bottle — that turns out to hold the clue to a legendary lost treasure somewhere in these very islands. The children are eager to search for it, but it soon becomes clear they are not the only ones interested in the treasure...