The Gwynns, a pleasant American couple, have lived outside York for the past fourteen years. Nesta, their only child, was born there and attends the local school. They seem ordinary enough and comfortable in their leafy suburb. But they have an astonishing secret unknown even to Nesta. One evening when she sees her father diminish and disappear into a stone lily pad in the garden pond, Nesta has to be told what she really is. Her parents are visitors from the planet Ormingat, sent to Earth to investigate life there. Now they have been ordered to return home. Nesta can`t take it all in, refuses to accept that she is not earthborn and finally runs away with the help of her best school mate, Amy.