On 28 Februry 1981, the Fourth Doctor and Adric landed the TARDIS by the side of a motorway to measure a police box. Air hostess Tegan Jovanka stumbled aboard the TARDIS on her way to work at London's Heathrow Airport. The TARDIS took off with her in it. The Doctor and the Master later confronted one another on the Pharos Project. The Doctor slipped and fell, and then regenerated into his fifth incarnation. (TV: Logopolis, PROSE: Divided Loyalties)
On the same day, the TARDIS materialised on a spaceship, where the Fifth Doctor and Adric found Urbankans and human androids. The Doctor saved the Earth from the leader of the Urbankans, Monarch, who had planned to poison humanity in order to steal equipment needed to travel in time. Having decided they would no longer have any place on Earth, the androids decided to pilot the ship to a new world and continue their lives there. (TV: Four to Doomsday)
Behind the scenes
This was one of very few instances in which a Doctor Who television story, namely part one of Logopolis, took place on the same day as its broadcast: 28 February 1981. Other such examples are 16 July 1966 and 26 June 2010.