The series' premise is that in 1980, Earth is being visited by aliens from a dying planet, who are abducting humans and harvesting their organs for their own bodies. The alien incursions may also be a prelude to a possible full-scale invasion. The series' main cast of characters are the staff of a secret, high-technology international military agency called
SHADO (an acronym for
Supreme
Headquarters
Alien
Defence
Organisation) established by the governments of the United Kingdom, the United States, the
Soviet Union, France, and Germany to defend Earth and humanity against the mysterious aliens and learn more about them, while at the same time keeping the threat of an alien invasion hidden from the public. SHADO has a variety of high-tech hardware and vehicles at its disposal to implement a layered defence of Earth. Early warnings of alien attack came from SID, the
Space
Intruder
Detector, an unmanned computerised tracking satellite that constantly scans for UFO incursions. The forward line of defence is Moonbase from which the three lunar
Interceptor spacecraft, that fire a single explosive warhead, are launched. The second line of defence includes Skydiver, a submarine mated with the submersible, undersea-launched Sky One
interceptor aircraft, which attacks UFOs in Earth's atmosphere. The last line of defence is ground units including the armed,
IFV-like SHADO Mobiles, fitted with
caterpillar tracks. On Earth, SHADO also uses two SHADAIR aircraft, a Seagull X-ray supersonic jet (e.g., in the episode "
Identified") and a transport plane (e.g., in "
A Question of Priorities"); a transatlantic Lunar Carrier with a separating Lunar Module (e.g., in "Computer Affair"); a helicopter (actually, a small VTOL aeroplane with large rotating propellers (e.g., in the episode "
Ordeal"); and a radio-controlled Space Dumper (e.g., in "The Long Sleep"). The Moonbase has hovercraft-like Moon Hoppers/Moonmobiles that can be deployed for transportation or reconnaissance. The alien race is never given a proper name, either by themselves or by human beings; they are simply referred to as "the aliens". They are humanoid in appearance, and the autopsy of the first alien captured reveals that they are harvesting organs from the bodies of abducted humans to prolong their lifespans. However, the later episode "
The Cat with Ten Lives" suggests that these "humanoids" are actually beings subject to alien mind control, and one "alien" body recovered was suspected of being completely
Homo sapiens, "possessed" by one of the alien minds—a concept central to the Andersons' previous Supermarionation series
Captain Scarlet and the Mysterons. Their faces are stained green by the hue of a
green oxygenated liquid, which is believed to cushion their lungs against the extreme acceleration of interstellar flight; this liquid is contained in their helmets. To protect their eyes, the aliens wear opaque
sclera contact lenses with small pinholes for vision. (The show's opening titles begin with a shot of one of these contact lenses being removed from an alien's eye.) The personal arms of the aliens resemble shiny metal submachine guns; these have a lower rate of fire than those used by SHADO. The aliens' spacecraft can readily cross the vast distances between their planet and Earth at many times the
speed of light (abbreviated and pronounced as "SOL"; e.g., "SOL one decimal seven" is 1.7 times the speed of light), but are too small to carry more than a few crew members. Their time on station is limited: UFOs can only survive for a couple of days in Earth's atmosphere before they deteriorate and finally explode. The UFOs can survive far longer underwater; one episode, "
Reflections in the Water", deals with the discovery of a secret undersea alien base and shows one UFO flying straight out of an extinct volcano. A special underwater version of the standard UFO design is seen in "
Sub-Smash". In flight, they are surrounded by horizontally spinning vanes and emit a distinctive pulsing electronic whine that sounds like a
Shoooe-Wheeeh! (produced by series composer
Barry Gray on an
ondes Martenot). The craft is armed with a laser-type weapon, and conventional explosive warheads can destroy it. ==Cast and characters==