Landmarks No.13

The War Games.
A landmark story for several reasons.
It is the final adventure for the Second Doctor, as well as companions Jamie and Zoe (save for future guest appearances).
It is the last story to be made and broadcast in black and white.
The Doctor's background is (partly) explained with the introduction of the Time Lords.
It is Terrance Dicks' first on-screen writing credit, after numerous rewrites behind the scenes on other stories.
Frazer Hines had originally intended to leave around the time of The Krotons (in one of the stories which it replaced), but he elected to stay on when Troughton informed him that he would be leaving at the end of the series - so that they could go together. Troughton had been increasingly frustrated with the quality of the scripts and the punishing work schedule. Wendy Padbury was invited to stay on and help bridge the gap to the new Doctor, but she decided to leave as well - allowing for a totally fresh start for the 1970 season.
The introduction of the Time Lords will have implications for the programme down to the present day.
There was no real reason to give away quite so much about the Doctor's background. The Time Lords could have turned up, solved the War Games and punished the Doctor - remaining enigmatic throughout, their actions unexplained. They and the Doctor could have refused to explain who or what they were - maintaining the mystery.
However, the Doctor's departure from their home planet, theft of the TARDIS and breaking of the non-interference laws are all set out. Whilst this provides explanation for the audience, it does begin the gradual demystification of the character and lead to future continuity constraints.
The Time Lord costumes will be referenced in The Sound of Drums - the young Master seen wearing one of the black and white robes.