Philip, Dinah, Jack and Lucy-Ann, all recovering from a bout of measles, are under doctor's orders to take a holiday while they recuperate. Learning that their friend Bill the police agent needs to go into hiding for a while, they persuade him to take them bird-watching in the remote islands north of Scotland. But it turns out they are not alone in the wild seas after all. Something secret and dangerous is going on — and those behind it are determined to ensure that no-one finds them out.