Doctor Who: 9 Worst Things The Master Has Ever Done
5. Series 3 Finale - The Year That Never Was
When the Master makes his debut in NuWho, he ends up forced into regeneration and fleeing the end of the universe. Stopping him as best he can, the Doctor 'traps' him back in the present day.
However, upon returning to the present day, the Doctor finds out that he has allowed the Master to lay the ground work for one of his most dastardly plans. Integrating himself in Earth society, using some mind influencing technology, the Master manages to become Harold Saxon, Prime Minister of Great Britain.
Long story short, the Master manages control of the Earth, turning it into a war machine to fight across the cosmos. Arguably the worst part of his plan isn't the subjugation of humanity, but the use of their future generations at the end of the universe to become the deadly Tocolafane, who 'removes' one tenth of the Earth's population.
During the year that is eventually unwritten, the Master keeps the Doctor (showing the true age of his body) as his pet, enslaves Martha Jones' parents and keeps Captain Jack as a prisoner. As Francine Jones, Martha's Mother, says at the end all those aboard the Master's flagship, the Valiant, still remembered the horror that took place, keeping it all very real, and an awful year on Earth.