The Enterprise is on a mission to convince the Halkan Council to allow the Federation to mine dilithium crystals on their planet. Kirk, Scotty, McCoy, and Uhura beam back up to the planet, but while doing so, an ion storm causes them to rematerialize in a parallel universe. There officers advance through assassination and the Empire plunders and pillages every planet they can find and destroys anyone that gets in its way.

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.