The Fallout fandom is about to have a LOT of arguments over one of the plot points in Far Harbor, Fallout 4's biggest DLC. Spoilers ahead! Far Harbor starts out simply enough: you need to find Kasumi Nakano, a missing woman who suspects she might be a synth. Searching for answers, Nakano ends up retreating to a synth colony far north, in a place called Acadia. There, you'll meet DiMA, the synth who looks a hell of a lot like Nick Valentine.
DiMA tells you that meeting Nakano is no problem; she's there of her own free will, as are all the other synths in Acadia. But, DiMA suggests, maybe Nakano isn't the only reason why you're in Acadia. Maybe something else, a small doubt, has brought you to a colony full of synths seeking answers. Here's how the encounter goes down, if you're curious:
DiMA poses a question that I'm sure has crossed most Fallout 4 player's minds at some point or another: is there any chance you could be a synth?
It is not an idle question. DiMA lays out all the different things that could be taken as proof of synthdom, such as the inability to recall early memories, and feelings of isolation. Since we as players can only experience Fallout 4 the day the bombs dropped, our experiences line up with that of synths perfectly by design. Our memory gap is enormous! There's so much about our own life that we do not know. Being a protagonist means you are also literally nothing like the other people in the world, too. Being a synth would explain so much, wouldn't it?
You might feel differently, of course. That's what makes this potential twist so good: there's no definitive answer. There is only doubt.
A twist like this could have been waffled easily — it's basically the sci-fi version of "IT WAS A DREAM ALL ALONG!" But the reason it works so well here is because of the groundwork that Bethesda laid in the main game. By telling you that anybody could be a synth, much of the Fallout 4 playerbase was overtaken with paranoia. One question I saw pop up time and time again was, "how can you tell if a settler is a synth?" Videos on YouTube exploring this question racked up hundreds of thousands of views, and slowly, "foolproof" methods of recognising a synth in Fallout 4 started floating around.
Synths have higher energy resistance, they claimed. Synths behave oddly, don't act like any of your other settlers. Put them into traps, construct elaborate tests in your settlements, players suggested. See how the NPCs react then. Worst case scenario, the settler dies. That's the most certain way of knowing, after all: does the corpse have a synth component? Congratulations, you found your witch.
Is the player a synth? Maybe. Maybe not. Whatever the answer is, I hope we never find out.