Jon Pertwee's fourth season as the Third Doctor kicks off with the show's 10th anniversary saga, "The Three Doctors." When a renegade Time Lord attempts to escape from an anti-matter universe, it requires the combined efforts of the First (William Hartnell), Second (Patrick Troughton), and Third Doctors to save the day. The Doctor and Jo's (Katy Manning) planned vacation on Metebelis 3 is sidetracked when they wind up in an alien sideshow of extraterrestrial lifeforms, in "Carnival of Monsters." In "Frontier in Space," the Doctor and Jo land in a future Earth starship and discover that the Master (Roger Delgado) is attempting to start an interplanetary war. Then, the Time Lord and his companion arrive on the "Planet of the Daleks," where they must stop an army of the malevolent beings from being unleashed upon the universe. And, finally, a Welsh village that's home to a new chemical plant, a deadly menace in a long-abandoned mine, and a malevolent computer with a mind of it's own all add up to a dangerous mission, in "The Green Death."