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First, I'd like to say that the movie's title has relatively little to do with the plot. It's about mathematics, but pi is not the primary mathematical concept in the film. Second, I want to say that I find it amusing having the movie's rating right next to the title almost makes it seem like part of a mathematical equation. Third, the movie is in black & white, and somewhat grainy. And I can't quite imagine it being filmed any other way.
Anyway... it's an interesting movie, though I feel like it would probably be more interesting to mathematicians than general audiences. And while I rarely think of myself as a "general audience," I'm certainly no mathematician. But you don't really have to understand advanced concepts to have a reasonable grasp on what's going on. There's this guy named Max Cohen, who believes nature is full of mathematical patterns, and... um... well, he tries to find patterns in the stock market. I have no idea what his actual job is, but there is some group (led by a woman named Marcy Dawson) that is interested in his talents. He wants nothing to do with them, until they deliver a classified computer chip to him, which should help in his work. He also meets a Hasidic Jew named Lenny Meyer, who's part of a group that's studying mathematical patterns in the Torah. And Max sometimes visits his former teacher, Sol Robeson, with whom he plays Go. And there are some people in Max's apartment building with whom he occasionally interacts, including a little girl named Jenna, who likes to give him math problems to solve in his head, and a woman named Devi, who acts friendly towards him. But Max is quite uncomfortable with most of these associations, because of social anxiety. Also he frequently gets blinding headaches, and seems to have hallucinations.
Well... Max is very close to finding some kind of number which is somehow very important, though it's not really clear why it was important. In fact, Max himself didn't seem to think it was that important, but... Sol had once been doing similar research into the number pi, until he gave up mathematics after suffering a stroke. But he'd found a 216-digit number, and apparently Max's number was that length, and Lenny's group was also searching for a number of that length. So... honestly, I don't feel like any of this made much sense, but apparently it's not a coincidence, even if everyone's research was done in different ways and for entirely different reasons. (The only one that ever provides an answer as to what their research was all about is Lenny's group, but I won't reveal what their goal was.)
And... I don't want to say any more about the plot. It's all rather strange, and it doesn't ultimately amount to much. But like I said, it was interesting. Sort of.