The Basics of Doctor Who:
All 13 Doctors created by @Aaroonio on Twitter

– The Doctor is an alien, known as a Time Lord, from the planet Gallifrey and has two hearts.

– The Doctor has a vehicle that can travel through time and space and change shape; but something went wrong and the TARDIS (Time and Relative Dimension in Space) is perpetually frozen to look like a 1950s police telephone box.

– The Doctor has an affinity for Earthlings, so generally, though not always, has one traveling along on adventures.

– The Doctor doesn’t remain in the same body, but “regenerates” when death is imminent (hence calling most versions of the Doctor by a numerical name.) This way various actors are able to play the Doctor and the show can continue. When it was first done in the 1960s it was a very big gamble, because they were seeing if you could continue to the same character despite acting and looking like a completely different person.

For a property that is constantly telling stories about change, these appear to be the unchangeable parts of The Doctor’s story.

(If you’re screaming at your screen that I forgot the sonic screwdriver, remember that item has never been a constant in any of the series, nor has every iteration of the Doctor had one. The First, part of the Fifth, Sixth and most of the time of the Seventh Doctors did not have a sonic screwdriver. The Twelfth Doctor also did not have one for a time and had sonic sunglasses which he continued to use even after getting a new sonic screwdriver.)