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!

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.

A woman awakens from a 10-year coma and begins telling the tale about her encounter with aliens, along with her boyfriend, while they were high on drugs. They toyed with the aliens, and the man apparently falls to his death from a bridge or building. The aliens drag him off. She has an accident and falls into the coma. She can't remember quite everything, but the old boyfriend is back, resuscitated by the aliens, and he drugs her so she tells him where the missing part of a device is. He activates what turns out to be a thermo-chemical bomb that will wipe England off the map when the four chemicals meet in a central chamber.