It's the series finale, so its main job is to tie up the recent story arc. This time round, we've been given a sort of finale-prequel episode - Utopia - which has already done much of the hard work. We know that Prof Yana is the Master, his identity hidden by a Chameleon Arch, and so this explains the Face of Boe's last words - "You Are Not Alone".
As the two-part finale begins, we see that the Doctor, Martha and Jack escaped from imminent attack by the Futurekind by fixing Jack's Vortex Manipulator. They arrive in London to find that the General Election has taken place and Mr Saxon has won - and Martha identifies the new PM's voice as that of the person who just stole the TARDIS. Thus, the Harold Saxon story arc - begun back in Series 2 - is explained.
Russell T Davies had always planned on bringing the Daleks back in the first series, and the Cybermen in the second. The third was earmarked for the Master from the outset.
The Master had last featured in the 1996 movie, played by Eric Roberts, in which he had been dragged into the Eye of Harmony. Here, it is explained that he was saved from this state by the Time Lords in order to fight in the Time War. He quickly ran away from this and hid himself via the Chameleon Arch, taking on the Yana persona.
Ever since he had first appeared back in 1971, in Terror of the Autons, the Master had been involved in some very odd schemes - ranging from the badly thought-out to the downright crazy. Initially, he seemed to ally himself with alien powers and it had to be pointed out to him time and again that they would turn on him as soon as they got what they wanted from him.
Once into his Anthony Ainley incarnation, he seemed to get crazier with each appearance.
RTD had to provide the character with some sort of motivation, and hit upon the idea that he didn't just act crazy - he actually was crazy. This would explain some of his more bizarre schemes.
He hit upon the notion of the constant noise in the Master's head - a never-ending drumming. This would prove to form part of a later story arc.
Fans believed the four beat drumming was based on the Doctor Who theme music, but RTD claimed it was based on his alarm clock.
The Master had often targeted Earth, purely because the Doctor was trapped there, and later just because he knew that this was the Doctor's favourite planet - so it made sense that the new incarnation would also set his sights on Earth and the human race.
Stealing and adapting the Doctor's TARDIS allows him to create a Paradox Engine - enabling the future humans (now Toclafane) to wipe themselves out in their own past.
The Toclafane derive from the planning stages of Series 1. At one point the BBC had angered the Terry Nation estate by assuming that they could use the Daleks without gaining permission and organising payments. The estate therefore decided to withhold agreement to use them.
Robert Shearman had to adapt his episode to feature an alternative villain - something known as the "Sphere". A single one would feature in the sixth episode, with a whole army of them in the finale.
As it was, the situation surrounding use of the Daleks was resolved, and the Spheres were held back for potential later use. That turned out to be the finale of Series 3.
With a younger Doctor in the TARDIS, RTD decided on a younger Master to contrast with him.
In the same way that the Doctor has always had a companion, it was decided that the Master should also have one - in his case a wife. This couldn't be the same kind of friendly mentoring relationship, however. The Master is clearly abusing his spouse.
The Master was an expert hypnotist, at least in the Delgado days, and so a form of mass hypnosis plays a role here. He is using a satellite network to boost his subliminal messaging to the entire planet.
UNIT feature - his old foes - and here they have a new toy: the Valiant.
This has all too obviously been inspired by the SPECTRUM Cloudbase from Gerry Anderson's Captain Scarlet.
Just before he flies out to join it, we see the Master at an airfield. He is wearing a black coat which has a bright red lining - hinting at Count Dracula's cape, but also one of Jon Pertwee's.
The scene with the Master watching the Teletubbies was inspired by a sequence in The Sea Devils where he was seen watching The Clangers.
Other references to the Pertwee / Delgado era include the phone conversation between the two Time Lords, where they discuss the old days and mention Axons and the like.
With the Time Lords all gone, and Gallifrey destroyed, fans never expected to see the Doctor's homeworld in the new series. However, here the Doctor reminisces about it and we get a flashback sequence in which we see the Citadel (based on the version which was first used by Doctor Who Monthly's news page). We also see some Time Lords clad in the robes and collars first designed for them by James Acheson in The Deadly Assassin. The sky appears to be an orange colour - taking us back to the description Susan gave in The Sensorites - though there it was supposed to be the night sky which looked that colour.
The same flashback shows us the young Master approaching the Untempered Schism - and he is dressed in the plainer black and white robes of the tribunal from The War Games Part Ten.
In Planet of Fire, the production team had contemplated confirming that the Master was the Doctor's brother - something which the original 1970's team had never planned. They had thought of making him another aspect of the Doctor - his darker side, not unlike the Valeyard.
A joke is made of the brothers idea, with the Doctor laughing at the notion.
Journalist Vivien Rook was named after real journalist Jean Rook, who had once written a critical piece about Robert Holmes' more horrific stories for the series.
The Master's speech to the country at the beginning of the first instalment opens with "Britain, Britain, Britain...", which is how Tom Baker usually began his Little Britain narrations.
He also starts another speech with "Peoples of the Earth, please attend carefully...", which was almost what the Ainley incarnation included in his blackmail message in Logopolis.
RTD did toy with giving the Master a new Tissue Compression Eliminator weapon, but thought that it would be one gadget too many, having already got the Laser Screwdriver and the Toclafane.
It is suggested that Jack becomes the Face of Boe. We had seen his home in the Boeshane Peninsula in the Torchwood episode Adam. It's never been confirmed but it would explain his great longevity and how he knew that the Doctor wasn't the last Time Lord and about Yana.
There is an unbroken run of episodes, should you choose to create a smaller story arc, comprising TW; Captain Jack Harkness, TW: End of Days, Utopia, The Sound of Drums, Last of the Time Lords, Time Crash and Voyage of the Damned.
Next time: Neighbours Meets The Poseidon Adventure...