The Master is a fellow Time Lord to the Doctor. He has had two regenerative cycles thanks to the High Council of the Time Lords.
The year that never was
In the Last Great Time War, the Time Lords themselves brought the Master back from oblivion in order to use him as a weapon in defence of Gallifrey. However, he deserted the instant the Dalek Emperor took control of the Cruciform as the sheer scale of the conflict seemed to frighten even the Master. He fled to the end of the universe and used a Chameleon Arch to become Human, remaining in the guise of the elderly Professor Yana. Martha Jones, who had traveled to this time period with the Doctor, recognized Yana's fob watch as similar to or the same as the one carried by the Doctor after he experienced the Chameleon Arch and unintentionally prompted Yana to open the watch. The Master returned in his old identity and attacked his assistant, killing her even as she shot him in the stomach.
Fatally wounded, though now aware of his identity, the Master regenerated (in one of the few confirmed accounts from his later life of him actually undergoing a natural regeneration) and escaped to 2000s Earth in the Doctor's TARDIS. There he assumed the identity of Harold Saxon and successfully ran for the position of Prime Minister. He then proceeded to take the Doctor prisoner and take over the Earth with the help of the Toclafane and a paradox machine he had made from the TARDIS. This timeline was reverted, however, when Jack Harkness destroyed the paradox machine. As the Doctor took him into custody, the Master was shot by Lucy Saxon, and collapsed in the Doctor's arms. Knowing that he and the Doctor were the last two remaining Time Lords, and knowing how badly the Doctor wanted to not be alone, he refused to regenerate, telling the Doctor, "I win." The Doctor burned his body on a funeral pyre, but a mysterious figure retrieved the Master's ring from the ashes.
The Master was resurrected by the Disciples of Saxon, a sinister cult who worshipped the Master. Lucy Saxon, however, had made a potion of death to counter the potion of life used to revive the Master. In the ensuing explosion, the Master escaped as a shadow. Once he regained physical form, he discovered that the interruption of the ritual had left bizarre side-effect. He was pale and ghost-like and even dyed his hair white-blonde in an attempt to be unrecognisable as Harold Saxon. His flesh would occasionally turn transparent, exposing his skull. In addition he had developed the ability to shoot bolts of artron energy from his hands and the power of flight, but using these abilities came at the cost of his life-force. Furthermore, he seemed more insane than ever. His previously urbane and sophisticated personality had been replaced with a frenzied bloodlust and a ravenous and insatiable hunger. He would kill everyone he encountered for little to no reason, laughed maniacally at the slightest occurrence and his behaviour bordered on that of a wild animal.
The Doctor found the Master and pursued him. They confronted one other and the Master used his psychic Time Lord abilities to let the Doctor into his mind, letting him hear the drums. The Doctor seemed profoundly disturbed and the Master escaped again. The Doctor still pursued him until the Master was kidnapped and taken away.
He was told to fix an alien gate called the Immortality Gate. The Master obviously jumped at the opportunity and fixed it. The Doctor and Wilfred Mott came into the room just as The Master entered the gate and Wilfred hid in an anti-radioactivity chamber of the power supply. The gate is found out to be a machine that changes the genetic DNA of the whole planet changing all Humans into his image. Everyone looked alike and acted alike. Only two humans escaped, Wilfred Mott, who was safe in the chamber, and Donna Noble, who was safe because she was part Time Lord.
The Master later realised that the Time Lords were returning through the discovery of a Gallifreyan diamond, sent through the time lock by Lord President Rassilon, and also learned that the drumbeat in his head was a link that the Time Lords could use to return to the material universe. Upon their arrival, the Time Lords undid his actions with the Immortality Gate, restoring the human race. The Master soon learned that Rassilon had placed the noise of drums in his head as a child to be used as a link to Earth. As Gallifrey appeared in Earth's sky on a collision course with Earth, the Doctor had to decide between killing the Master and killing Rassilon. If the Doctor killed the Master the link would be broken and everything would return to normal, but if he killed Rassilon, the Master indicated the Doctor would become the new leader of the Time Lords as Rassilon is Lord President. After the Doctor notices The Woman, he turns on the Master to apparently kill him, but instead orders him to "get out of the way." The Master, realizing what he intends, dives to the side as the Doctor destroys the Whitepoint star with a shot from Wilfred Mott's gun, severing the link as the diamond was acting as an anchor. As Rassilon prepared to kill the Doctor, the Master, in an act of vengeful and possibly heroic self-sacrifice, used the last of his life-force to disable the deranged Time Lord leader before both the Time Lords and the Master vanished in a burst of white light. His final action saved the Doctor, and helped save the universe by stopping the Time Lords' plans, returning them and Gallifrey to the Time War. It is currently unknown if and how the Master survived. It is likely he was sucked back into the Time War with the other Time Lords and died when the Doctor destroyed Galifrey.