Plot

SHADO - an acronym for Supreme Headquarters Alien Defence Organization - is located beneath a supposed film studio in the English countryside and run by Commander Straker, who poses as a film producer. When an Unidentified Flying Object attacks a plane in which Straker's deputy Colonel Freeman is travelling it is shot down and investigated. The body of an alien is found and discovered to have transplanted human body parts, seemingly from a trio who disappeared in nearby woods a decade earlier. It is part of the alien plan to take over the Earth to study and imitate humans.—don @ minifie-1

In 1970 a young couple and chaperon come across a grounded UFO. The young man (Peter Gordeno) takes pictures of the craft and the three are ambushed by orange-suited aliens. The young woman cornered and the man is shot. Still in 1970, a USAF Colonel (Ed Bishop) lands at a small airfield and is met by government Rolls Royce and motorcycle outriders. The Air Force officers brief a minister but while en route are attacked by a low flying UFO. The Rolls crashes, destroying the evidence, but the colonel is thrown clear.

The time is now 1980. The young man is Captain of the Sky-Diver and pilot of Sky-1, a submarine launched fighter aircraft, a team of astronauts, male and female, man a lunar base, an ever-vigilant Space Intruder Detection satellite monitors space, and Colonel Straker is now a movie producer, and secretly in overall command of SHADO from a complex built beneath his studios.

It looks like the aliens cannot reproduce themselves, so they have found out a way of becoming immortals in practice by means of constant organ transplants. The water in which they encapsulate their heads allows them to breath in an alien atmosphere (the Earth's) and gives them a bluish skintone. The aliens have been kidnapping humans for some time, in order to extract the organs which will they will use later on on themselves to remain alive.

One alien has survived a crash. Nina Barry (Dolores Mantez) has found out a way to track the UFO's even though when they travel faster than speed of light. The SHADO has succeeded in destroying some UFOs, and they have even trapped an alien alive. Somehow, that alien understands the humans, but absolutely refuses to talk or communicate in any way. When the alien dies, he is analysed, and his heart is found to have belonged to the sister of one of the SHADO soldiers. That girl had disappeared some years ago, and nobody knew what has happened to her until now. The brother is told the truth, but he has to invent a weird lie to tell their parents.