The Gwynns, a pleasant American couple, have lived outside York for the past fourteen years. Nesta, their only child, who was born there 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. There is a time limit on the Gwynns return to Ormingat. By forcing them to miss this deadline, Nesta will be condemning her parents to staying on Earth for the rest of their lives. She is totally bewildered by her feelings of both love and hate for them and in her dilemma she seeks out Mrs Dalrymple (surrogate mother of Thomas Derwent from Space Race). Like Space Race, Earthborn has a gripping climax as to whether her parents - with the dubious help of the police - will track Nesta down before time runs out for them.