When the Stewarts spend a sunny, frosty December day at London Zoo, they're enchanted by one small penguin. At the delight of young Imogen and Arthur, Mrs Stewart insists the penguin must come and stay with them whenever he likes. But not one Stewart expects the penguin to turn up at their door that evening, rucksack labelled Einstein on his back... The family's new feathered friend helps Arthur to come out of his shell and makes massive demands on Imogen's amateur sleuthing. But together they must find out why Einstein came to them and they must keep away from the mysterious white-coat man. And Einstein can't stay forever, can he...?