UFO's plot revolved around the gradual invasion of Earth by aliens. The aliens had been kidnapping Earthpeople for some time, and (spoiler warning) harvesting their organs for the aliens' own uses. A multinational defense force called SHADO (Supreme Headquarters Alien Defense Organization) was created to detect and combat the alien menace, which they did through the use of a base on the moon, orbiting satellites, submarines, fighter planes, etc. SHADO's main base was located in a secret underground facility beneath a movie studio, and Commander Ed Straker, whom the public thought to be merely the head of the studio, was actually in charge of the coordinated Earth defenses. The infiltration of the aliens, however, was kept from the public at large (a plot point previously seen on the American TV show The Invaders).
Since it was an Anderson production, UFO featured lots of hardware, including the moonbase and the fighter planes launched to intercept the aliens' spacecraft. The Anderson team had largely stayed together through a variety of series, and on each one the quality of the special effects - especially the miniatures - was extremely high. In addition, the show used a number of sets and props created for a film called Journey to the Far Side of the Sun (aka Doppelganger), which also featured a number of the same actors. In each episode - and this was a trend started years before in Anderson series - there would be an extended scene showing off the hardware, such as pilots readying for takeoff, platforms being lowered into underground bunkers, spacecraft moving into position, etc.
Coming as it did in 1970 (with filming beginning in 1969), UFO was a product of its funky, super-cool times. The soundtrack is psychedelic, the designs are way-out, and the episodes progress in a fashion typical of action-adventure series of the time. The events of the show were actually set ten years in the future, in 1980, but the clothes and architecture and technologies are from a different world. One thing many people remember about the show are the purple wigs worn by the female technicians aboard the moonbase - which was apparently a purely aesthetic decision, since no explanation for them is offered on the show.
The series didn't fare well in Britain and lasted only one season - but then, many of the Andersons' productions, whether well received or not, lasted only a short time. Reruns two years later proved fairly popular in the United States, however, and plans were made to film a second season; when this fell through, the preliminary designs were reportedly used by Anderson to pitch a new series to the American market which became Space: 1999. Later on, pieces of UFO episodes were spliced together to form a 30-minute TV movie entitled Invasion: UFO.
Although not as well remembered as such Anderson shows as Thunderbirds or Stingray, UFO remains a fun, funky piece of late-60's adventure programming. It stands as one of the more memorable British offerings of the period, alongside such cult favorites as The Prisoner or The Avengers.