Under threat from a mysterious green skinned, liquid breathing alien race, Earth set up SHADO (Supreme Headquarters, Alien Defence Organisation) under the leadership of former USAF commander Ed Straker. Not wishing to cause wide spread panic amongst the public, SHADO's secret headquarters was buried deep below the Harlington-Straker film studios on the outskirts of London. From here, Straker, assisted by Colonel Alec Freeman , Captain Peter Carlin, and Colonel Paul Foster, ran operations and kept in touch with the planets first line of defence which was situated on Moonbase, where response to an alien attack came in the form of typical Anderson gadgetry, namely the Interceptor Spacecraft. If this line of defence were breached then SHADO could still call on Skydivers, nuclear crafts capable of either underwater or aerial combat. The organisations female staff were clad in white cat suits and had purple hair, the most famous of them being Gabrielle Drake and Wanda Ventham.

The series was developed by Gerry and Sylvia Anderson and Reg Hill in the autumn of 1968 while production of the mostly forgotten puppet series The Secret Service was still in progress. The Anderson's were also completing production on their first live-action feature film Doppelganger. Around that time ATV supremo Lew Grade agreed to finance UFO to the tune of £100,000 per episode. Filmed over a period of 18 months the series used the facilities offered by three studios starting with the MGM British Studios in Boreham Wood followed by the Anderson's own Century 21 Studios in Slough and finally to ATV's Elstree Studios. Towards the end of 1969 MGM decided to close down the Boreham Wood studios and production on UFO came to an enforced halt. By that time only 17 episodes of the 26 had been completed and it wasn't until May 1970 that the production of the full run was finally finished.

UFO had all the elements for a successful series; a good strong storyline, plenty of action and the wonderful Anderson models. Where the show failed was in the uncertainty of programme schedulers who couldn't make up their mind if the series should be aimed at children or adults, and as a result it meandered between Saturday morning and late night 'graveyard' slots until, eventually, a planned second season was cancelled to make way for the Anderson's next project, Space 1999.