Parts: 2 (45-minute)
Writer: Paula Moore
Airdates: 5 January 1985 - 12 January 1985
Production Code: 6T
Creature Feature: Cybermen, Cryons
Story: Deep in the London sewers, a pair of workers find that a recently-maintained sewer tunnel has suddenly and mysteriously been walled off. One of them goes to have a look around the adjacent tunnels, and disappears. When the second worker follows him, he encounters something horrible and deadly lurking in the shadows...
The Doctor, meanwhile, decides to take a break from repairing the TARDIS circuits to give Peri a holiday - but his plans are interrupted when the TARDIS picks up an alien distress beacon coming from Earth in 1985. Peri reminds the Doctor that not all aliens want to take over the Earth, so the Doctor determines that, if possible, he will find and help the stranded creature.
However, as usual, things are not what they seem. The distress call has been sent by the Doctor's old adversary Lytton, who is in the process of carrying out a diamond heist - but, as his team infiltrates the sewers, they encounter a base of Cybermen. From their base on the planet Telos, the Cybermen intend to change history. Next year, in 1986, the Cybermen's home planet Mondas will attack Earth and be destroyed in the process...but the Cybermen intend to redirect Halley's Comet and destroy the Earth, thus assuring the survival of Mondas and their race...