It's Christmas, and the inhabitants of Porridge Hall are suffering from the winter blues. The weather is horrible, Mrs Robinson's ill, and, to top it all off, the Brogan's dog, Old Blanket, dies. So when the children hear the tale of a dolphin sword which brings good fortune, they're determined to track it down . . .
Unfortunately, however, Mrs Robinson is in decline, so Ant and Perry are shipped off to stay with Aunt Mabel, while Beany and Sun Dance are left with Mrs Brogan. Aunt Mabel, it transpires, is a silver-haired eccentric living in a menagerie - and not at all what the children expect. Then Sun Dance discovers a burglar, and the mystery of Aunt Mabel is revealed . . .