Landmarks No.17

The Three Doctors.
To open the 10th Anniversary Season, the previous incarnations of the Doctor are brought together to meet the current TARDIS incumbent.
We visit the Doctor's homeworld for the third time.
The creation of the Time Lords is fleshed out a little as we learn of Omega's involvement in giving them their time travel capability.
We now take multi-Doctor stories a bit for granted, but this was the first, and arguably the best. Later such stories feel more shoe-horned. The 50th Anniversary story will feature more than one Doctor, but they are only going back to the last (still extremely popular) one, plus adding a new, unseen, Doctor into the mix.
Barry Letts had been approached by fans many times, suggesting a story which would bring back Hartnell and Troughton, but he had always dismissed the idea. With the 10th Anniversary, the idea suddenly seemed too obvious to ignore any longer.
We first visited the Time Lord homeworld in 1969's The War Games, and it was briefly glimpsed again at the beginning of Colony In Space. We spend quite a bit of time there in this story, though only get to see the main control area. Despite the frequent references to the Doctor's people over the last couple of years, the planet is still unnamed at this point.
Omega turns out to be a solar engineer from the dawn of the Time Lords. He detonated a star to create the energy source that allowed them to begin their time travel experiments. Unfortunately he perished in the attempt - or so the Time Lords thought.
Another first for this story is that it marks the Brigadier's (and Benton's) first trip in the TARDIS. It seems surprising that, despite working at UNIT HQ for about three years, neither had ever been invited aboard before to take a look round.