Parts: 4
Writer: Eric Saward
Airdates: 8 March 1982 - 16 March 1982
Production Code: 6B

Doctor: fifth
Companions: Adric, Nyssa, Tegan

Creature Feature: Cybermen

Story: When an archaeological team is attacked and killed by an unknown assailant, the sole survivor - Professor Kyle - seeks help from a military group. Kyle leads them through a series of caves toward the cavern where her team was attacked while, on the surface, their lifesigns are monitored via a scanner. But the scanner is experiencing strange flares of light, and whatever is causing the flares is also disrupting the power in the caves.

After a member of the party is injured during a blackout, she is helped to get back to the surface...but a pair of shadowy figures silently follows their path. On the scanner, another sudden flare wipes out their lifesigns, as well as that of Snyder, a woman who had gone down into the caves to help them. When another pair from the team goes back to learn what happened, they find only Snyder's nametag, floating in a puddle of ichor.

While Adric stays in the TARDIS, the Doctor, Nyssa and Tegan take a walk through the cave system. They find Kyle's party on edge, and they also find a mysterious silver plate set into the cave wall. When they try to examine, they are attacked by the same two shadowy figures. As everyone runs for cover, the Doctor reveals that they are androids - built to protect whatever is hidden behind the hatch. The androids are servants of the Cybermen, who have plans for the destruction of Earth. But for the Doctor and his companions to stop them, they will have to pay an enormous price.