from the Doctor's past. During this trilogy (begun in the serial Mawdryn Undead, and concluding with Enlightenment), the enemy was the Black Guardian, who was last encountered by the fourth incarnation of the Doctor at the conclusion of The Key to Time saga in the 1979 serial The Armageddon Factor. The story also saw the return of the White Guardian, who had also not been seen since 1979. Production Conception and writing After penning a number of radio and TV scripts, including episodes of Crossroads and Waggoner's Walk, Barbara Clegg submitted a story idea to Doctor Who script editor Eric Saward, an acquaintance from the BBC drama department. Interested in writing for the series, Clegg had been inspired when distant relatives had stayed with her and demanded constant entertainment during their visit, basing the character of the Eternals upon them. Initially titled The Enlighteners, her submission involved ships racing through space that, with the addition of the Black Guardian sub-plot, eventually evolved into the story as screened. Saward and series producer John Nathan-Turner liked Clegg's ideas, and they commissioned the script in September 1981. The first episode was delivered by Clegg in October and the three following episodes arriving in January 1982. The serial was now scheduled to conclude a three-story trilogy featuring the Black Guardian, and Clegg duly wrote the recurring characters into her scripts. By May 1982 there were problems with a script by Pat Mills, Song of the Space Whale, which had been intended to open the Black Guardian trilogy. Mills' script was eventually dropped and the production team considered moving The Enlighteners forward in the season to replace it, necessitating considerable re-writes. Peter Grimwade was eventually commissioned to write Mawdryn Undead to replace The Song of the Space Whale, and The Enlighteners was confirmed as the fifth serial of the season. The first draft of Part One did not contain any of the material concerning the Guardians, and Turlough was a peripheral figure, with the script focussing on the relationship between Marriner and Tegan. With pre-production underway, Saward changed the story title to Enlightenment in September 1982, a title he felt was more enigmatic. Saward also rewrote portions of the script pertaining to the story-arc, particularly the final confrontation scenes at the end of Part Four. Peter Moffatt had been originally scheduled as the serial's director, but following the problems with the Space Whale script he