Plot Summary:The Doctor is captured by the Cybermen and taken back to their home planet. There he discovers the source of the signal.The Cryons, original inhabitants of the Cybermen's world, survive underground and they want the Cybermen gone. The Doctor learns the full extent of the Cybermen's plans. Not only are the Cryons threatened but so too the very future of the human race. This is no war of conquest, but total destruction.