The show ran from 1963, the day after Jack Kennedy’s asassination, to 1989. It got tired and the special effects, which had never been all that special, were made to look even worse in comparison to the breakthroughs used in movies, which were numerous times more expensive than the BBC could afford. However, there was always a nagging demand for it to return but of course commissioners of programmes like to show that they have new show ideas rather than bringing back something that was an idea from 1963.
The special effects of movies over the years came down in price such that enough of them could be afforded by television. This, combined with ceaseless nagging led to it being brought back in 2005.
The hero (the Doctor) sometimes encounters past enemies from as far back as the first series in 1963. He can regenerate (new actor) when it is plot and actor convenient. Every couple of series he encounters The Master, an original enemy from the early days. The Master can regenerate, indeed The Master’s last regeneration resulted in a change of sex to The Mistress, not really a word to be often used in a show meant for all ages. I like Missy, she has some evil and witty lines. You may know of the actor who currently plays The Doctor, Peter Capaldi. You may know of the actress who plays Missy, Michelle Gomez who was in The Book Club, Green Wing and Bad Education.
Strangely enough the BBC hold the copyright to this :-
The Metropolitan Police sued the BBC and eventually in 2002 lost. It is worth millions in terms of numerous categories of product sales over the last 53 years and continuing.