Well, we've kind of covered some of this ground in "Mode of Production," but just to recap: the film is mostly straightforward: there's one storyline, and it involves following John Nash's progress from socially awkward outsider to revered super genius.
However, we did say "mostly," and there is a catch: a lot of what we see in John's storyline is actually hallucination. John's hallucinations, to be exact.
So, there technically are parallel realities happening in the film, but we don't get clued into that fact until halfway through. Better late than never, though, eh?