Doctor and Commander
Disclaimer: I own neither Doctor Who, nor Discworld.
The Tardis materialised on the brass bridge. It did not gain anything more than token interest from Morporkian citizens going about their sometimes dubious daily business as it looked unlikely to do anything interesting such as explode. A man in long brown steeped out with a cheery smile and began to survey the unparalleled view of the turgid, and in summer, green, with occasionally a very strange tree growing on the surface. He escaped the notice of all in the crowd except for one man. Commander Vimes narrowed his eyes and proceeded forward until he was level with the man.
"Why are you here this time?" he asked abruptly
The Doctor turned and grinned at him cheerfully. "Just visiting, always just visiting."
Vimes fixed him with an accusing stare. "Last time you were here, it was at that damn opera in Uberwald. Before that, you were in the Klatchian foreign legion and the Leshp business was going on. Whenever you turn up, there is trouble on the horizon."
The Doctor feigned surprise "I don't find trouble it finds me! Anyway, how did you spot me in the foreign legion?"
"Well, the crossbows of an entire regiment mysteriously misfiring gets noticed and you were there with that daft suit, according to eyewitness reports. Another thing that happens when you're around is that lots of mechanical things tend to break, each breakage heralded by a mysterious buzzing noise and the appearance of that strange thing you call a screwdriver" Vimes said dryly "and you were playing for the Klatchians in that football game Carrot organised. I have never seen someone run so fast in my life with possible exception of Rincewind!"
"Ah, how is the old coward? I have never seen anyone save a planet in such strange and cowardly ways. The poor man just wants the quiet life from what I can see."
"Judging by his track record, a quiet life is something he is never going to have. Why did you join the foreign legion anyway?"
The Doctors face darkened. "You of all people commander should understand why I would want to forget."
Vimes nodded. Yes he understood alright. From what he had heard that man had lost far too many friends, and had the burden of an incredibly long life, mostly travelling alone. He wondered what it would be like.
The Doctor abruptly walked away from Vimes and with sadness in his eyes, said "Goodbye Commander, you probably won't see me again, so it is goodbye forever. Give my regards to your wife and Havelock, Sybil, Carrot and Angua would you. Oh, and tell Carrot to get on with it, the universe will end before he proposes at this rate. Also tell Ponder Stibbons to speak in plain language and get a girl. Life will probably be a lot easier."
Vimes snorted "Why you refer to the Patrician by his first name I do not know and probably never will understand. Goodbye Doctor."
As the sun set over Ankh-Morpork, the TARDIS dematerialised, heading to snowier climes and somehow even stranger people, while Vimes went to pass on the messages. Somewhere, somewhere unreachable save for by two entities, Death remembered the future and said OH BUGGER.