The first story to not have a companion for the Doctor. Its also the first story to not feature at least one human.
Our first fully fledged story about the Time Lords. Many of the elements about the Doctors race originate here, things like the iconic collars and the Seal of Rassilon also make there first appearance. Despite this at the time many fans were annoyed by the portrayal of the Time Lords. In their previous brief appearances they had appeared almost godlike but here they were mainly bickering politicians. However writer Robert Holmes knew a story of such perfect all powerful beings wouldn't be interesting and so explored the idea of their greatness stagnating after ages of isolation. This is also the story that created the 12 regeneration limit.
After 3 years the Master had finally returned. Of course with the death of Roger Delgado it was decided that he would be "regenerate" like the Doctor. Its never made clear what version of the Master Delgado played and thus whether this version is a new emaciated incarnation or the same but simply on his last legs. We see this happen again in a new way in the End of Time when Simms Master is resurrected but requires energy to survive.
This version of the Master, played by Peter Pratt, would go on to appear again in the tv series as well as various audio stories, but played by Geoffrey Beevers. Who incidentally was married to Caroline John, who played the 3rd Doctor's companion Liz Shaw until her death in 2012.
The cliffhanger to ep3 infamously pissed off Mary Whitehouse for being too scary for children. Her campaign to clean up TV forced the BBC to edit it for the repeats.
The intro is the first time the show used narration.
We see actor Bernard Horsfall for the 3rd time! Previously being Gulliver in The Mind Robber and then in The War Games as one of those Time Lords. He also starred in Jon Pertwee's Daleks greatest hits story, Planet of the Daleks.
The concept of the Matrix here is far more imaginative then that presented in that overrated film.