Spock is unusually restless and ill-tempered. After he throws a bowl of soup at Nurse Chapel, Kirk insists on an answer and Spock reveals that he is in the early stages of Pon Farr, the Vulcan time of mating that occurs once every seven years. He must return to Vulcan to mate or he will die. Kirk goes against Starfleet and heads toward his friend's home planet, but the Vulcan ritual doesn't go as planned and Spock must soon fight Kirk to the death.

A team of archaeologists travels to the planet Telos, the home of the Cybermen, to excavate the tombs of the long-extinct cybernetic race. At least, it's assumed that they're extinct. The Doctor isn't sure that the tombs should be disturbed, but he soon comes to suspect that some of the "archaeologists" have an agenda far more dangerous than the team leader, Parry, suspects. In order to find out what the sinister Kleig and Kaftan are really up to, the Doctor helps the archaeologists to enter the tombs - where they find that the Cybermen are only dormant, not dead, and that Kleig and Kaftan intend to revive them. But they have underestimated the power of the Cybermen, and only the Doctor can save the archaeologists from being transformed into Cybermen themselves.