The current incarnation of the Master is played by John Simm, a mad man who would do anything to destroy the Human race and the Doctor. In series 3 of Doctor Who the Master become known as Harold Saxon, the Prime Minister of Great Britain. The Doctor, Martha and Captain Jack managed to stop him by reversing time creating the year that never was.
The Doctor thought he’d stopped the Master for good but part of him lived on. When the Master was resurrected he wiped out the Human race changing history for ever which ultimately interfered with the Time War.