The master

The Master, a.k.a Seraph, is a villain and member of the Time Lords, the same species as the Doctor. He was born in the Rassilon Era, and is approximately the same age as the Doctor. He is often shown to be insane, although what was thought to be a dominant symptom of this was in fact a beacon placed by the Time Lords to escape the timelock placed on Gallifrey during the Time War. When the Phalanx campaign came to the Kasterborous system, the Time Lord army battled at the Capitol and a majority of it was destroyed. The Master was shot during this battle, but the wound was not fatal so regeneration was not necessary. One of the few characters who has never had contact with a mutant or mutate of any sort, those called the Isolations, he fled the Time War before the Avengers and the X-Men were called on to the front lines. He escaped to the year 100,000,000,000,000, where no Time Lord had ever ventured. Before leaving the War he used the Chameleon Arch, a device of his and all TARDISes, to metamorphose into a human. He was downloaded with the name of Professor Yana (which is later proved to be the acronym, You Are Not Alone, the last words of the Face of Boe to the Doctor) and helped the last remaining humans to escape the heat death of the universe. During this period, his head hurt whenever elements of Time Lord lore were mentioned, and later discovered a fob watch covered with Gallifreyan symbolism (the medium in which the essence of a Time Lord who had used the Chameleon Arch was stored). Upon opening it, the entire identity of the Master returned to his head. His alien assistant Chantho, a Malmooth, was electrocuted by him but managed to fatally shoot him before dying. The Master locked himself inside the Doctor's TARDIS, while the Doctor, Martha and Captain Jack were trapped outside, surrounded by the Futurekind, the devolved remnants of humanity. The Master proceeded to regenerate inside, becoming a younger man. Before the Master leaves, the Doctor sonics the TARDIS console so that the Master can only travel between the year 100,000,000,000,000 and the present day.

The trio return to the present day to discover that the Master has somehow been around eighteen months prior to when the TARDIS was supposed to land. He has assumed the identity of Harold Saxon, and has recently won the election. He has pioneered the Archangel Network, a group of satellites around the Earth. Claiming to be a mobile phone service, they are actually a telepathic machine grid, enabling the Master to influence the decisions of the entire population of Britain. On the day of his election, he announced to the world that an alien species, the Toclafane, had made contact with the Earth. The Doctor recognized that the Toclafane was the name of the Time Lord equivalent of the boogeyman. When the first few arrived, the president of America had travelled to the UK to meet them. Upon saying that they are looking for 'their Master', the Master revealed himself and orders that the Toclafane execute the President. The Master had captured the trio and used his laser screwdriver to artificially age the Doctor 200 years, leaving him an old man. Martha had been given a teleport by Captain Jack, and used this to escape. The Master announces the end of the world to the camera, before millions of Toclafane emerge from a temporal rift in the sky above the Valiant.

A year later, Martha had travelled around the world and returned to Britain to meet Tom Milligan, an authorized human. The Master had turned the Earth into a dystopia, and millions of nuclear missiles were being created to wage war on the universe. Martha and Tom go to see a woman in possession of electricity, now an incredibly rare luxury. Martha showed the two a video of a Toclafane being deactivated by a strong electrical surge. When Tom attracts the attention of one, while chasing him it is electrocuted. They examined the Toclafane, which is revealed to have a living human head inside. Martha realizes that the Toclafane are the humans from the year 100,000,000,000,000 horribly deformed. When the Master kills Tom, Martha returns to the Valiant with the Master. They bring Jack with them, and they all come to the very room where it started. Before the Master kills Martha, she tells him that she just travelled the world, telling people about the Doctor. On the Earth below, all the people begin chanting the Doctor's name, and amplified by the Archangel Network, the Doctor frees himself. He levitates surrounded by psychic blue energy, and disarms the Master with telekinesis. The TARDIS had been turned into a Paradox Machine, necessary for the Toclafane to kill their ancestors, but Jack destroys it. Time begins to reverse itself to one year ago, just before the Master took power. They all remain in the room. Lucy Saxon, the Master's human wife, shoots the Master fatally. The Doctor cradles him as he dies, refusing to regenerate. The Doctor collapses into tears as he becomes, truly, the last of his kind.