The Enterprise responds to a distress call from the planet Mulour, only to find the entire civilization has been destroyed. Then a small probe with incredible weaponry and defense capabilities attacks the Enterprise. After Kirk identifies himself, it agrees to be beamed aboard to make contact. The probe, Nomad, was sent out to explore the far ends of the galaxy, but it has altered and is now dedicated to seeking out and sterilizing life forms. Only the fact that it believes Kirk is its creator is keeping it from destroying the Enterprise... and it starts to see through the deception.

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.