Rigid continuity and firmly nailed down rules of the universe have never been a priority for Doctor Who. Its the absolute enemy of telling a good story and, after fifty-one years and dozens of different writers, gathering together every single rule that the programme has established, and creating some kind of definitive set of guidelines would be like painting the Forth Bridge. While some aspects of Doctor Who lore have persisted for long enough that theyre almost always followed (like the general idea of fixed and in-flux points in time), pretty much everything else is taken as read but quietly shunted to the side if it gets in the way of a decent story. This also applies to the stories and the tone. Thanks to Doctor Who being (for lack of a better term), an anthology series, practically everything changes week-on-week. In the past few weeks weve zigzagged from organ harvesting robots, to an ethically layered journey through a Dalek, to a swashbuckling Robin Hood adventure. And this is one of the many reasons that Doctor Who persists: its almost impossible for it to get boring and repetitive since pretty much every genre, setting, and tonal approach is fair game without breaking the programmes internal logic.