Talking about Predestination without confidently, fearlessly giving away plot points that shouldn't be given away is enormously hard. The best thing would be to go in totally cold, not even knowing what short story it's based on (having read the story - it is not at all obscure, though calling it "well-known" might be a stretch - I was unfortunately able to get ahead of the movie pretty early on when I recognised it, which was kind of disappointing), so if you trust me, please go find it; it's crawling across the U.S. right now, and also can be found on several VOD sites. If you only trust me a little, how about this to sweeten the pot: no matter what you end up thinking about it, it's worth seeing because in it, you will find Sarah Snook giving an immaculate, wholly-formed Star Is Born performance of the first order. Gimmickry, raging sentiment, immensely subtleties of facial expression and gesture, showy vocal work; whatever you thinking Great Acting is, she's got some of it for you. The role won the Australian Academy Award for Best Actress against some weighty contenders, and boy, it's not the half of what she deserved.

So I'll assume you either have seen the movie, or don't care (but no worries, I'll keep spoilers firmly limited to the first third). Predestination is, to begin with, a film by Australian filmmaking brothers Michael and Peter Spierig, whose last film, 2009's "what if?" vampire story Daybreakers, felt like a peerless example of a once-in-a-lifetime concept that simply didn't have enough done with it in the execution. No such hedging applies to Predestination, which has an equally hearty concept, but sees it through to its conclusion in a way the previous film didn't. That's partially because it's an adaptation of Robert Heinlein's "All You Zombies-", one of those mid-century science fiction short stories that seems to exist primarily so that the author could show all his steps in solving a problem and disingenuously calling it a narrative; all the Spierigs had to do was minutely copy the story and just like that, fully-realised narrative. And that is, in fact, mostly what they've done, even when it leaves their movie snagged on ideas that work as asides in a short story and feel like conspicuous dropped threads in a movie. The biggest change is to add a bit of a "stop the terrorist" thriller spine that extends the plot far enough beyond the "taa-daa, I've revealed everything" gesture that it feels like an actual story with actual stakes and themes. Which makes it, whatever else it is, one of the too-rare movies to have thoroughly improved upon its source material in the act of adaptation.

The plot, meanwhile, is designed with a few confusing fake-outs along the way (it opens with the kind of in medias res scenes that annoyingly telegraphs that we'll see bits of it again later, and then it will make sense; a gimmick that I have long grown tired of), but in its basic for, it goes like this: a man (Ethan Hawke, the lead in Daybreakers), who has just received a new face after being badly burned, has for uncertain reasons set himself up as a bartender in New York in the early 1970s. One night, another man comes in, and after some fencing that could be seen as basic bartender chitchat but feels like goading, this second man offers to tell the most amazing story that the bartender has ever heard, betting a fresh bottle of booze that it will knock his ass off. And then begins his story, of the time when he was a young woman named Jane (Snook), who grew up an orphan with a distinct sense of being Not Like Other Girls; at a certain point, to make ends meet while traveling into space, she attempts to get a job as one of the space-courtesans serving the astronauts growing lonely and horny on year-long missions (it's details like these where scrupulous fidelity trips up the movie a bit). This doesn't pan out, she gets involved with a man who leaves her pregnant, and the birth results in a shocking discovery: Jane is of biologically indeterminate gender, and to save her life after this turns out to have made giving birth a near-fatal ordeal, the doctors have elected to turn her into a man.

It's right about then that it all clicked for me: not that Jane was the man in the bar (that's made clear so early that it's unfair to call it a twist), but that Snook had been playing him all along. Prior to that, the odd vocal cadences and carriage that went into the performance seemed just like part of the pile-up of mysteries that Predestination was indulging in during its opening act; it literally didn't occur to me that it was a woman playing a man until the slow progression of make-up joining the two end points (the film boasts some stellare make-up effects) made it impossible to miss. Snook is that good at playing the chameleon, figuring how the personality of a depressed man in 1970 is different from a brittle, whip-smart woman in the early 1960s, and exploring those registers so fully that it seems like she's playing two entirely different characters; and yet, the evolution between the two of them feels totally natural and correct.

And that's just at the level of the central gimmick: at all points in her performance of both iterations of Jane/John, Snook's carefulness in letting the right emotions slip out at the right time is so flawless in such a sustained way that even if this was the most sedate, genre-free kind of story, her performance would be hardly any less exemplary. There's nothing about any moment she's onscreen that isn't mesmerising; even Hawke seems dumbfounded, happily handing her each and every scene they share together - and it goes into some place where they share some really weird scenes together - without compromising his own rather nuanced and tricky performance, but also without showboating.

The two leads are so faultless in their performances, and Snook is so commanding in every scene, whether as a worn-out man or a fierce woman, that it's easy to lose sight of the fact that Predestination isn't actually a character study at heart; it's a carefully laid-out exercise in narrative mechanics, which asks multiple times, to varying degrees of explicitness, "are our actions the result of choice, or are they (wait for it) predestined? And if a certain choice is immoral, does the fact that it is predestined obviate our need to resist making it?" A bit dry and theoretical, sure; this does have its roots in '50s literary sci-fi, and dry theory never found a genre and a period that suited it better.

And, indeed, the dryness of it does tend to diminish the film's impact as more than a precisely-described puzzle with really urgent thematic questions that are more fun to think about then they are to apply to one's own life. The Spierigs do yeoman's work keeping the film's narrative clear and visually connected to itself, and the confidence of the filmmaking helps to keep things purring along any time the more logically-inclined viewers might start to feel the need to tug at the film's central impossibility. But it's very arid, in a lot of ways: impeccable but sterile, and only the fact that Snook and Hawke are in there, insisting on the warm, sloppy humanity of their characters allows Predestination to feel like a relatable story and not just an extraordinarily well-framed exercise.

But then, this is a movie, and movies do have actors, and maybe that was the point all along: take Heinlein's clinical chilliness, and make it into a rich human story by throwing some rich humans at it. I'm not sure it's quite enough to make Predestination an actual great piece of cinema, but it's absolutely enough to make it terrifically compelling viewing: a film with ideas and feelings that are inseparable from each other is nothing to discard lightly, even when it feels like it might be, at some level, a giant put-on.