Doctor Who - The Five Doctors
A twentieth anniversary special featuring the Doctor in all five of his regenerations.

Why are all five Doctors being removed from their separate time-streams? Who is the enemy they will have to unite against? What will become of the Doctors when the battle is over?

We have travelled a long way with Doctor Who. The Five Doctors gives us the chance to turn the clock back and meet some old friends — and some old enemies.
Fiction, media tie-in, Doctor who (fictitious character), fiction, Science fiction, Time travel, Space and time
Terrance Dicks
Terrance Dicks is an English writer and former script editor, best known for his work in television and as the author of many popular children's books during the 1970s and 1980s.

His best known association during his television career was with Doctor Who, working as a writer and - from 1968 to 1974 - the program's script editor. He also adapted many of the televised stories into novelisations for Target Books.

Source: Wikipedia