In the conclusion of the four-part story "The Deadly Assassin," the Doctor (
Tom Baker) has successfully proven that the Master (Peter Pratt) was behind the assassination of the President of Gallifrey. Alas, the Master has it within his power to destroy all the Time Lords -- one of whom has secretly been helping the villain throughout this adventure. Written by Robert Holmes, "The Deadly Assassin, Episode 3" first aired on November 20, 1976. ~ Hal Erickson, All Movie Guide