Doctor Who has been so popular for more than 60 years now because of how versatile it is. Episodes can be musical adventures about the Beatles, space hijinks with weird aliens, or big multiversal sagas full of timey-wimey stuff. One of the most common — and interesting — approaches for a Doctor Who story is to take an original sci-fi story and find a way to insert the Doctor.
As onetime showrunner Steven Moffat said in an episode of Doctor Who Confidential, “You know you’ve got a good idea for a Doctor Who episode if you think ‘Well, I’ve just blown that feature film idea forever, haven’t I?’” 15 years ago, a perfect example of this set up Doctor Who for an epic mid-season finale, taking a classic sci-fi premise and doing it in a way only Doctor Who can.
