Who hasn't dreamed of a holiday free of grown-ups and rules? For the Packet children that dream suddenly comes true when their mother's friend Cynthia, who lives in the south of France, offers the family the use of two ancient caravans in a field near her house. Their mother has just got a job, so the problem of what they should do in the long summer holidays has been unexpectedly solved.

There is ten-year-old Rose, who is dreamy, loves painting and is devoted to her guinea pig. There is her elder brother Simon, who likes machinery, conducting experiments and finds stick insects more interesting than guinea pigs. There is their bossy cousin Linda who is always quarrelling with Simon, and her sensible fourteen-year-old brother, James, who does his best to keep the peace.

But there are problems: one is Rose's plump black and white guinea pig, Porky, which she doesn't want to leave behind. Then there is the sinister Mademoiselle Sourire employed to escort them to their destination by boat and sleeper. It is Rose, with a secret of her own to conceal, who first wonders if the mysterious French woman is all that she claims to be.

The holiday is idyllic. After settling them in and showing them the shops, Cynthia and her husband Pierre are happy to let them look after themselves. So they revel in the warmth, in the smells of the herbs and lavender fields, in swimming in the lake, having picnics and above all in not being treated like children.

It is a confusion over cases-which could happen to anyone really and their contact with Mademoiselle Sourire which turns this peaceful holiday into a great adventure, involving them in a struggle with an international gang of thieves and putting Rose in terrible danger.