In the year 1980, a secret military organization named SHADO (Supreme Headquarters Alien Defence Organisation) becomes operational. Its purpose is to defend the Earth from Aliens who've been coming to Earth and kidnapping humans. It is believed that these Aliens are a dying race, and are transplanting human body parts to extend their own lifespans.
SHADO's main headquarters is hidden beneath a film studio near London England. SHADO also has a base on the moon (Moonbase) and a fleet of submarines (Skydiver). Incoming UFO's are initially detected by an Earth orbiting satellite named SID (Space Intruder Detector). An initial interception attempt is made in space with three Interceptor craft launched from Moonbase. A second attempt is made in the Earth's atmosphere with a fighter plane (Sky One) launched from Skydiver. Landed UFO's are tracked down with ground Mobiles.
SHADO is run by the extremely dedicated Commander Ed Straker, and is overseen by General James Henderson. Straker's seconds in command are Col. Alec Freeman, Col. Paul Foster, and Col. Virginia Lake. Moonbase is often commanded by Lt. Gay Ellis or Lt. Nina Barry. Skydiver is often commanded by Capt. Peter Carlin or Capt. Lew Waterman.
The episodes are typically about SHADO's attempts to thwart the Alien invasions, and the various Alien strategies to disable SHADO or kill its commander. Recurring themes include maintaining SHADO security, keeping SHADO properly funded, and the effect that the secrecy has on the personal lives of SHADO's operatives.