As if Doctor Who fans haven't already had their minds blown by the revelations from Chris Chibnall and Jodie Whittaker's 12th series, here comes series franchise heavyweights Russell T. Davies and Steven Moffat to tag-team another pretty reveal. During a dual interview between the two screenwriters for the series' monthly magazine, it was revealed that 2008 episodes "Silence in the Library" and "Forest of the Dead" actually introduced a future version of the Doctor (played during this time by David Tennant) disguised as Colin Salmon's satellite-with-feeling Doctor Moon.
Apparently, Moffat emailed Davies a brief outline during production that mapped out the past and future of the Doctor and River Song (Alex Kingston). In the draft, Moffat explains that he sees River as both a wife and widow and that their story both begins and ends at the same point: "Somewhere in the terrible future, on a battlefield, the 45th Doctor dies in her arms and makes her the same promise she once made him – it's not over for you, you'll see me again. So River buries her husband and off she goes to have lots of adventures with his younger selves and confuse the hell out of them."
When River ends up in Library Planet's data core, the reality of her never seeing him again sets in. Of course, even at such a down moment as that, she can't help but critically start to wonder "why anyone would call a moon 'Doctor.'" For Moffat, the Doctor's dying move felt like one that fits with both the character and his relationship with River: "The Doctor worrying that she'll get lonely in the library, and popping his dying mind inside a moon." For Davies, the idea has changed the way he views the episode: "Every time I watch that story, I think, it's him, it's the Doctor, and no-one knows!"