The UFOs are flown by the aliens who are believed to want to invade Earth.
Not much is known about the specifications of the alien craft except that they are fast and difficult to stop.
S.H.A.D.O. - Supreme Headquarters Alien Defence Organisation - have a satellite in orbit for advance warning of alien incursion, and a team on the moon and on earth for dealing with them.
The series was designed by Gerry and Sylvia Anderson and first broadcast in the UK in 1972.