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.