He took up his post as script editor at a time the show was facing unprecedented crisis, following the firing of previous Doctor Colin Baker and the acrimonious mid-story departure of his predecessor Eric Saward, who had fallen out with producer John Nathan-Turner. Cartmel therefore started his first script-editing job with a script he didn't commission, by writers he didn't get on with, for a Doctor they hadn't cast yet.
Cartmel sought to reinvigorate the series and take it in a new creative direction. He discarded the graphic violence and toned down the Continuity Porn that had characterised the previous era, and returned to Nathan-Turner's approach of recruiting new, young writers to the show rather than re-using those who had previously written for it.
He also sought to restore the element of mystery to the character of the Doctor from the show's early years, which Cartmel felt had been lost over the course of the show's run. To this end, he and some of his writers (mainly Ben Aaronovitch and Marc Platt) threw around ideas for the Doctor's backstory which would have revealed that much of what had previously been established about the Doctor was wrong, and he was in fact a much more powerful and mysterious figure than had hitherto been revealed. While hints of this were dropped throughout Cartmel's final two seasons on the show, its cancellation in 1989 came before the revelations could be fully played out onscreen. This went down in fan legend as the "Cartmel Masterplan", though Cartmel and friends insist it was never really a "master plan", simply some cool ideas they threw around between each other.
Despite Cartmel's efforts, the ratings decline that Doctor Who had suffered under his predecessors could not be reversed, not helped by BBC executives pitting the show against Coronation Street, leading to the show's cancellation at the end of 1989. At the time, he and the rest of the writing staff were already putting together a farewell season for McCoy and Sophie Aldred, whose contracts were due to expire in 1990. Despite the show's cancellation, he has continued to contribute to the Seventh Doctor's career in various parts of the Doctor Who Expanded Universe, in particular having an influence on the Doctor Who New Adventures, which continued and intensified the morally-ambiguous characterisation of the Doctor that he had introduced. (Though notably, "his" Masterplan doesn't feature at all in his novels, with Marc Platt actually taking that plot to a conclusion in Lungbarrow).
After Doctor Who, Cartmel was headhunted for the position of Script Editor on CASUAL+Y for a year, where he continued his efforts to make more relevant, socially-aware television. He also brought a number of Doctor Who writers with him, including both veterans from the TV series and New Adventures writers doing their first television work. Since 2010 he has also been writing a series of prose detective novels, The Vinyl Detective, about a record collector who is regularly forced to become an amateur sleuth.
