18 January 1969: broadcast of fourth episode of The Krotons. The Doctor makes up some sulphuric acid; Zoe uses it to poison off the Krotons, and Jamie uses it to destroy the Dynotrope.
18 January 1975: broadcast of fourth episode of Robot, which is the first I remember watching all the way through. The robot starts disintegrating people, but the Doctor manages to destroy it with metal-eating virus.
18 January 1982: broadcast of first episode of Four to Doomsday. Rather than Heathrow, the Tardis lands on a spaceship controlled by Monarch and inhabited by humans from four different eras of history.
18 January 1983: broadcast of first episode of Snakedance. Rather than Earth, the Tardis lands on Manussa, home of the Mara; Tegan is possessed by it and gets away from the Doctor and Nyssa.