Steve runs away from home, having argued with his father, and hides in a slate mine where his friend Paul finds him and brings him back. However, on returning to their village the two boys find the place completely deserted. Or so they think.

The Earth is threatened by an alien race who kidnap and kill humans and even animals and use them for their body parts. In the year 1980 S.H.A.D.O. (Supreme Headquarters Alien Defence Organization), a highly secret military organization, is set up in the hope of defending the Earth from this alien threat. This organization operates from a secret location beneath the Harlington-Straker film studio in London. S.H.A.D.O. also has a base on the moon with Interceptors as well as an early warning satellite that detects inbound UFOs (called S.I.D.), and operate a fleet of submarines. General James Henderson and commander Ed Straker have a team of highly trained and well equipped females and males to battle the regularly incoming hostile UFOs.