With their mother and father away at a conference, JULIAN, DICK and ANNE are to spend their holiday with the mysterious UNCLE QUENTIN - a brilliant scientist but something of a tartar, by all accounts. What's more, the family don't know anything about QUENTIN's daughter GEORGINA - what will she be like? GEORGINA (or GEORGE), meanwhile has problems of her own. Her beloved TIMMY has been disturbing QUENTIN from his work, and QUENTIN has ordered her to get rid of the dog. Determined not to lose TIMMY, she asks the local fisher boy ALF to look after him. JULIAN, DICK and ANNE arrive in Kirrin, and are immediately confronted with the open hostility of a bad-tempered GEORGE. She wants nothing to do with them, and becomes angry when they tease her about the fact that she looks like a boy. However, she softens when the others are dutifully impressed by the fact that she 'owns' Kirrin Island - and she agrees to take them over there on her boat to explore the old castle. TIMMY joins them, and a tentative friendship is established.
That night a terrible storm raises an old shipwreck from the bottom of the bay and pitches it upon the rocks. The Five explore, and manage to retrieve an old wooden box. GEORGE informs them that this is her great great great grandfather's wreck; and that the cargo of gold ingots that the ship was carrying has never been recovered. Could the box lead to buried treasure?