Landmarks No.22

The Deadly Assassin.
A controversial great leap forward in our understanding of the Time Lords, and therefore the Doctor.
Up to now, the Time Lords had appeared only occasionally. Despite some squabbling in The Three Doctors, and their penchant for making the Doctor carry out the odd task for them, they were remote, supremely powerful beings. From the War Chief's reaction, they seem quite god-like.
Some fans were quite horrified to see them presented in this story as being as weakly human as we are - inflicting torture on prisoners, being stupid, duplicitous, homicidal and physically frail - prone to dodgy hips etc. That the critic-in-chief was the President of the DWAS showed the depth of feeling this story engendered.
We take reinvention for granted these days. Look at the number of times comic book heroes are rebooted these days (both in print and on the big screen) - either thrown down new paths or started afresh with a new origins tale.
Robert Holmes doesn't actually reinvent the Time Lords. He takes what little has been seen already and uses is this as background to a political thriller. The Time Lords do bicker over protocol in The Three Doctors - despite the Universe being on the brink of destruction. They are hypocritical - aloof from the rest of the Universe but having the Doctor do their dirty work for them. The spotless deities were only ever in our imaginations.
Now we really know why the Doctor left Gallifrey.
The story is also significant for having no regular companion - although Spandrell simply fulfils this role temporarily.
It also finds a way of getting round the death of Roger Delgado to reintroduce the Master, without simply recasting. Things are left for a future producer / writer to bring back the character in a new form.
This might also - possibly - be the first time the Doctor has set foot on Gallifrey since he stole the TARDIS. We only assume the final episode of The War Games is set there. It could just as easily be some neutral world, or a space station like the one from the Trial season.