Five Rounds Rapid!
First there was Bret Vyon...
The Daleks' Master Plan
Then he met the Second Doctor and changed his name along with his job title.
The Five Doctors
Liz Shaw and the Third Doctor soon followed
The Silurians
As, apparently, did a modelling career
He tried to keep up with the Fourth Doctor's costume changes by shaving his moustache
Terror of the Zygons
Many adventures, and a short spell of amnesia later, he's teaching at a minor public school when along comes a Doctor (that's Doctor number Five).
Mawdryn Undead
His history with Six, however, has to wait a while to start
Dimensions in Time (Comic Relief 1993)
I have just finished listening to The Spectre of Lanyon Moor, for something like the sixth or seventh time, it is - to paraphrase the Doctor - quite excellent.
"You know what they say about old soldiers, Doctor."
Battlefield, with the fabulous Jean Marsh
"Every now and then I'm asked to do a spot of undercover work, and, every now and then, I say yes."
Step forward Brigadier Sir Alistair Lethbridge-Stewart
Sarah Jane Adventures