Freeman takes over temporarily while Straker deals with a woman apparently determined to investigate what SHADO is all about.
A mysterious dome is found under the sea, that at first looks like the Moon crashed into the ocean. Inside the dome, they find duplicates of SHADO HQ personnel, using non-alien technology - cassette tape recorders - to imitate SHADO instructions to Moonbase and Skydiver.
Foster is suspected of being a security leak, but during the trial, Straker and gang discover that Foster's apartment was bugged and shut down the disreputable business that is selling the spy gear.
Two of the SHADO people, dining at a fancy restaurant, begin to recall this event after one of them sees a diner who was part of Dalotek. A private company installed a facility on the moon which discovered an alien device that disrupted Communications between Moonbase and SHADO on Earth.
Reported missing, presumed dead, after a UFO incident which has left SID disabled, astronaut Craig Collins turns up alive and well. But as an operation to complete repair work on SID is planned, Colonel Lake and Colonel Grey discover that Collins is not the man he used to be...
A UFO imbues three young people with incredible powers. One blows up a radar station, a second steals fingerprints and tries to get aboard a Skydiver sub, blowing it up. The third, a woman, tries to blow up SHADO HQ but the UFO that is feeding her power has now been destroyed.
The body of an Alien, recovered after a UFO attack on Moonbase, reveals some startling new evidence which overturns all SHADO's previous theories about the Aliens. For Interceptor pilot Jim Regan, the news has tragic consequences as he becomes possessed by the mind of an Alien held in the body of a Siamese cat!