The Zygonites are aliens who fled from their home planet Megalatta when it was destroyed by its neighbouring planets. As a result of radiation their females were rendered infertile and they now feed themselves entirely by enjoying the distress of others. Their leader, Zygon who has strong telepathic powers, plans to propagate a hybrid race by mating with human teenagers.

Amanda lives with her husband Pete and their twin 15-year old daughters, Cathie and Rosie, in the New Forest near Lyndhurst. Cathie is raped by an alien and falls pregnant with a hybrid baby that resists all efforts to abort it. Fortunately for the human race Amanda herself is a telepathic hybrid having had a previous life on the planet Alpha and is able to enlist Alphas help to thwart Zygons plans on no less than three occasions.

In her struggles Amanda is ably assisted by her friend Michael Woodman who as Megalon, king of Megalatta, was once her deadly enemy, but now with the aid of humanizing fluid, lives a relatively peaceful life with his wife Sonja and twin sons, running a farm near Brockenhurst. Unlike humans, both have the means to travel to Alpha in virtually no time at all.