"A new body at last!" - The Master after stealing the body of Tremas
The Master originally known as Koschei, like the Doctor is a time lord from the planet gallifrey. At first the Master was friends with the Doctor they even both went to the same school together, but one day when the Master had to stare into the time vortex to prove himself he went insane and quickly became a renegade Time lord, a threat to Gallifrey and the Doctor.
The Master in his aged body
He made his first major move on Earth, helping the Nestines using a weapon called a tissue compression eliminator he creates an army of Automs, but luckliy the Doctor stops him. Later the Master hijacks a missile which he plans to use it to stop a peace conference but again the Doctor manages to stop him. After a series of failed attempts made by the Master he is captured by Unit.
He is soon imprisoned on a island, but he takes advantage of the guards who allow him to do what he likes, in his cell with some electronic equipment he helps the Sea devils to try and reclaim the Earth from the human race but he fails. Later he joins forces with the Daleks in order to start a war between Earth and Draconia, but yet again the Doctor stops him.
Melkur, his current tardis
Soon the Master has run out of his current regeneration cycle and now resembles a decaying monster, soon he returns to Gallifrey where he shoots the President of Gallifrey and frames the Doctor of the assanation, so he could take control of the Eye of harmony and trying to become the Keeper of traken to extend his life, although he is stopped he takes the body of a Time Lord called Tremas and regenarates thus giving him a new life of regenarations. Now with new life he attempts to control the universe with his destruction, after a series of failed attempts, the Time Lords decide the Master should help the Doctors in the Death zone at Gallifrey, sadly known of the Doctors trust him, but a group of Cybermen do but they end up being destroyed. Later the Master is killed by being burned alive in front of the Doctor, but survives and joins another renagade Time Lord, a female known as Rani untill the Doctor intervenes. Soon the Master tried to gain control of Gallifrey by desposing the councill and making himself the leader however he fails.
Soon the Master seeks help from the Doctor as he is on the run from the Cheeta people, but he fights the Doctor and after a battle he vanishes without a trace.
As the Time war begins with the fight against the Daleks, the Time Lords revive the Master and give him a new cycle of regenarations so he could fight the Daleks, but the Master soon flees's the war through time and uses a Time lord fob watch to turn him into a human. The war ends with the death of the Time lords and most of the Daleks, the Master and the Doctor are now the only Time Lords left alive.
The Master and the Toclafane
The Master is know a kind old profesor known as Yana in the year 1 trillion trying to help humans escape to Utopia, at one point the Doctor and two companions Jack and Martha arrive and meet Yana completly unaware of his true identidy, untill Yana open his fob watch and becomes the Master again. After getting shot by his former assistant he regenarates and steals the Doctor's tardis to Earth 2007 where he becomes Priminister under the name of Saxon and marries a woman called Lucy. The Master quickly stops the Doctor and Jack and unleashes a army of sphere like beings the "Toclafane" which are humans from the future twisted and turned into sphere like creatures, they kill most of the human population starting with President Winters.
In the end the Master is shot and the Toclafane are erased from history and time also reverses which leaves the population unaware of what happened and no memory about the Master. Although the Master could regenarate he refuses in order to have beaten the Doctor. The Doctor then gives the Master a decent funiral and burns his corpse. As the Doctor leaves a strange female hand pulls out the Masters ring out of the ashes. The woman turns out to be one of the Master's disciples. She along with the rest of the Master's disciples take a sample from Lucy the Masters wife and ressurect the Master, but Lucy intervenes his ressurection and his body is ripped open and constatnly discharges energy which he learns to use as a weapon, but he is soon captured by a millionaire who wants to use him to get the Immortality gate ready so he could give his daughter eternal life, but instead the Master uses the gate to transform every human on earth (minus Winston and Donna) into copies of himself creating the Master race. Soon the Master obtain a white point star which is normally found on the now destroyed Gallifrey, he uses it to create a link between Earth and Gallifrey so the time lords can return he is meet by Time Lord president Rassalon, the Master hoped to turn the Time Lords into himself aswell but Rassalon reverses the process and the humans turn back to normal, Rassalon then prepares to destroy earth in order to save Gallifrey, the Master asks to join him but is rejected. Then the Doctor shoots the Immortality gate which closes the link thus sending Rassalon and the Time Lords along with Gallifrey beack into the Time war to their deaths, Rassalon decides to kill the Doctor before he too dies. The Doctor accepts this but the Master instead saves the Doctor and fires his raw energy at Rassalon and is sent with him into the Time war where he is probably killed.