In the fourth episode of the six-part story "Frontier in Space," Jo Grant (Katy Manning), traveling companion of the Doctor (Jon Pertwee), experiences some strange and disturbing visions. Meanwhile, the commissioner of Sirius 4 reveals himself to be the Doctor's old nemesis, the Master (Roger Delgado), who has been using his hypnotic powers -- and an army of Ogrons -- to foment a war between the planets Earth and Draconia. Written by Malcolm Hulke, "Frontier in Space, Episode 4" originally aired on March 17, 1973. ~ Hal Erickson, Rovi