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The third, fourth and fifth stories of the twentieth season were conceived by John Nathan-Turner and Eric Saward as a trilogy reintroducing the Black Guardian and his opposite number the White Guardian - characters devised by former producer Graham Williams as part of his concept for the Doctor's quest for the Key to Time, as seen in season sixteen. They would be played, as before, by Valentine Dyall and Cyril Luckham respectively.
Nathan-Turner decided that during the course of the Guardian trilogy Nyssa should make her exit from the series and a new regular be introduced to take her place. (He had actually considered having her written out during the course of the previous season, but this had been objected to by Peter Davison - who considered her the best-suited of the three companions to his Doctor - and so the producer had ultimately elected to have Adric killed off instead.)
The new companion devised by Nathan-Turner was Turlough, a young man of mysterious background and suspect motivation who would be substituted for another schoolboy character already included in writer Peter Grimwade's story Mawdryn Undead. The actor chosen by Nathan-Turner to play Turlough after extensive auditions was Mark Strickson, who accepted the role in preference to an alternative one that he had been offered in the BBC's hospital-based drama serial Angels. His fair hair was dyed red for the part as Nathan-Turner thought that he would otherwise look too similar to Davison in long shot.
This trilogy was to be the last time that the awesomely powerful Guardians featured in Doctor Who.