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!

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...