1 Mercutio: An alternative theory of time travel says that events are fixed, and it’s impossible to change anything.
2 Mercutio: Will only thought his grandfather was Robert Arden, but all along it was whichever person takes his place and fathers his mother.
3 Mercutio: In which case whatever happens is the correct thing, and we don’t need to do anything at all to “fix” history, because it can’t be broken.
4 Shakespeare: I vote for that one.
4 Ophelia: Time travel isn’t a democracy!

I mentioned the idea of fixed time in the context of time travel back in the annotation of strip #2042.

Interestingly, this is one of the most tractable forms of time travel to use if you're playing a roleplaying game that allows time travel. Changing history gets really messy and it's easy to lose track of what players are doing. But if history is fixed and can't be changed, there are certain constraints. It may sound like the players won't be able to do anything at all, but you have to remember that things are not always as they seem.

In this case, as Mercutio points out, we thought that Robert Arden was Will's grandfather, but actually it could have been anyone, and the historical record was inaccurate. So if this was a roleplaying game, the PCs would be free to remove Robert Arden and replace him with anyone. As long as the historical record shows (falsely) that it was Robert Arden. So their mission might be to replace him and doctor the church records.

You can get away with a lot if you take this approach to time travel and history. Whatever you do, you just need to make sure the historical record matches what we remember.