One of the concepts that has taken root in the “Doctor Who” universe is that of “fixed points in time” — a clever way to say that some events can’t be changed because they are crucial to the fabric of the universe … yada, yada, yada.
It’s a storytelling technique that explains why even River Song can’t kill Hitler to spare us the horrors of World War II or Mount Vesuvius will destroy Pompeii no matter what (or because of any) actions the Doctor takes.
Now, Doctor Who: Adventures in Time and Space The Roleplaying Game (at least the 2009 version by Cubicle 7 that I have) doesn’t directly address Fixed Points by name. But in concept, it explains that even if time travelers succeed in messing with the timeline at crucial points, it’s likely another time traveler or powerful entity (at some point) went back and restored it (or prevented you from changing it in the first place) — which explains why things can’t be changed.
And while this is explained to the audience as high Time Lord stuff that’s not to be meddled with, the fact is it’s a nifty constraint imposed on the writers of the show to keep the episodes focused on character-driven stories, which is the heart of the series anyway.
Now, what holds true for showrunners such as Russell T. Davies and Steven Moffat, is good advice for GMs — whether you’re running a time traveling adventure that involves the TARDIS or some other cast of characters.
The fun of playing a tabletop rpg with all that timey-whimey stuff anyway is meeting historical figures from the past, battling monsters or robots from the future, using your wits more than blaster weapons, and rallying back after certain defeat (and a whole lot of running at points in between).
After all, killing Hitler isn’t the point (and in “Doctor Who,” not killing anything is really the point). But if you can stop aliens from giving advanced tech to Hitler’s war machine, that’s good. And if you stop those bug-eyed monsters from attacking or killing a great mind from earth’s past — say an Einstein or Ben Franklin — then that’s a fair day’s work. Companions are judged on how well they adapt to a given situation and find a solution that rewards those in trouble. (And if you stop clanking garbage cans that shout “Ex-ter-min-ate!” repeatedly, then alls the better).
Keep the action moving. Find opportunities for mischief-minded characters to romp around all “Scooby Who,” and lastly, present them with an “Ah-ha!” Solution to their crisis.
So, as a GM, the advice is to steer clear of Fixed Points, and instead, offer adventures that help the characters themselves develop as good, faithful, rebellious/resentful little companions, saving the universe one Sonic Screwdriver at a time. After all, once the TARDIS lands and the doors swing open, the only person who knows for certain what’s outside is you, the GM. And that is an opportunity that is not to be missed.