– 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.