The production team decided around the end of January 1980 to have three of this season's stories linked by a running theme. The theme that Bidmead came up with, setting it out in detail in a note dated 12 June 1980, concerned the dangers encountered by the Doctor and his companions after the TARDIS accidentally passes through a Charged Vacuum Emboitment (CVE) from their own universe, N-Space, into a completely different and rather smaller one, E-Space.
One function that the production team wanted the E-Space trilogy to fulfil was to provide a vehicle for the writing out of Romana and K9 and the introduction of a new companion to take their place. Lalla Ward was contracted for just the first twenty episodes of the season as she had agreed with Nathan-Turner that it would then be time for her to move on, while K9's exit had been decreed by the producer as he disliked the character and felt that it provided too easy a solution to the problems that the Doctor encountered.
The new companion devised by Nathan-Turner and Bidmead was Adric, a youthful 'Artful Dodger' type. The actor chosen by Nathan-Turner to take the role of Adric was Matthew Waterhouse, a young Doctor Who fan who had been working as a BBC clerk. He had been suggested to the production office by the BBC's internal Casting Advisory Service and been judged by Nathan-Turner, Barry Letts and director Peter Grimwade to be the best of all those who auditioned.