The Enterprise responds to a distress call in the L370 system and discovers that something has destroyed almost every planet in the system. They also find its sister ship the U.S.S. Constellation, which has been badly damaged. The captain of the Constellation, Commodore Matt Decker, says that his crew evacuated to one of the pulverized planets and was lost, and the attacker was a giant robot "doomsday machine". Kirk is stranded on the Constellation when the doomsday machine attacks again, and the shell-shocked Decker tries to take command of the Enterprise to lead a futile attack.

The Doctor returns to a Tibetan monastery which he hasn't visited for 300 years, only to be accused of murder. The legendary Yeti, normally timid and elusive creatures, have suddenly become vicious and savage, and many monks and visitors to the monastery have been killed. The Doctor faces suspicion from the monks and from the British explorer Travers, who's determined to prove that the Yeti exist and initially fears that the Doctor is from a rival expedition. Eventually the Doctor discovers that the savage Yeti are robots, built by an alien Intelligence who has possessed one of the monks. The Intelligence claims that it only wants physical being - but in fact, its new physical form will consume the entire Earth unless stopped.