The unfortunately loaded baggage that Side Effects carries with it, something it does not deserve but has to deal with anyway, is the question, "if this actually ends up being Steven Soderbergh's last theatrically-released feature film, does it make a suitable cap to his career?" Like any pointless rhetorical question, the answer is both "yes" and "no", though "no" comes a lot closer to winning out than most of us would have preferred. Not because Side Effects is a bad movie at all; simply that it quite unconcernedly, in a low key, one more film in the groove already etched by recent Soderbergh efforts Contagion and Haywire, an exploration of genre film mechanics, perfectly fine in its way, but hardly a summing-up; of course, the very idea of a summing-up, career-ending movie is so contrary to Soderbergh's longstanding habit of going in the unexpected direction wherever possible, that maybe Side Effects is the only kind of swan song he'd ever have.

Side Effects is a movie that starts out by being about one thing, and midway through turns out to have been about something else entirely. The first thing is a rather profound movie about depression, in which 28-year-old Emily Taylor (Rooney Mara) can't cope with the release of her husband, Martin (Channing Tatum), from prison, where he's served four years for insider trading. She attempts to kill herself by driving into the wall of a parking garage, which is what brings her to the attention of Dr. Jonathan Banks (Jude Law, better than he's been in ages), a psychiatrist who immediately puts her on a regimen of antidepressants, and when those don't end up working right, tries out a different pill, and so on, with the advice of her old therapist, the pharmaceutical shill Dr. Victoria Siebert (Catherine Zeta-Jones). The second thing, I cannot even begin to suggest, other than to tell you that it is a thriller, and not at all an anti-Big Pharma thriller, which is in a sort of vague way what the opening of the movie seemed to be heading towards, with its implication that psychoactive medicine is perhaps indiscriminately and arbitrarily over-prescribed, or at least mis-prescribed. That message ends up having nothing at all to with the actual point of Side Effects, which has more to do with self-destructive obsession, and people watching in disbelief as the apparatus of the 21st Century works with the stately precision of a Swiss clock to destroy every tiny scrap of happiness in their entire life.

It also tends to outright deny that what was going on the first half of the movie is what was going on, and that's more than a little frustrating: because it was such a lovely depiction of depression as a feeling of constant flatness and meaningless forward movement, with Mara giving what is, by a country mile, the best performance of her career to date, as Soderbergh (working as his own cinematographer) shoots everything in uncomfortable, even disorienting close-ups, not just faces but inanimate objects, to really drive home the sense of insurmountable closeness and claustrophobia of the main character's emotional state. Certainly, the film captures, in script, performance, and visual, the feeling of depression far better than anything that the recent Silver Linings Playbook does with Jennifer Lawrence's character, to name one example.

But then it scraps it all in a mid-film twist that complicates and deepens things on a level that makes me actually realise, for the first time in my life, what it must have been like to see Psycho when it was new, minus the transgressive violence. It's hard to say exactly what Soderbergh and screenwriter Scott Z. Burns (working with the director for the third time) were up to with this shift, because there are many possibilities, most of them to do with forcing us, in the audience, to confront what it is that we expect from our movies, why we make certain connections with certain characters, and how a savvy filmmaker can used that against us - Side Effects does not tell one single lie, though it surely does feel that way after a little while.

At a certain level, all of this is just in service of making one hell of a thriller, in which double-crosses fly by madly, in which the protagonist races ahead of us and we have to catch up only to find that all along, we actually knew just as much about the plot, only we weren't being encouraged to make the same wild but successful guesses. The third act, a chain of subterfuges that none of the principals seem to grasp fully, is constructed by the characters in such a blind way that it resembles those poker variants where you have to bet on cards that you aren't permitted to see. I mean this to be a compliment, incidentally; nothing serves to make a tense situation in a movie even tenser by demonstrating to the audience's satisfaction that not even the people onscreen have any idea what's going on.

But after Haywire, I don't want to accuse Soderbergh of making a simple thriller, even if it's a crackling one, and so we return to the question of why he and Burns so thoroughly trick us and rewrite the rules of their movie completely at the midpoint. All without actually changing the texture of the movie at all. Soderbergh manages to make largely the same aesthetic choices function differently: instead of close-ups emphasising the suffocating feeling of depression, they're close-ups that push us right up into the face of a mind in anti-rational freefall, for example. Thomas Newman's extraordinary score, an unclassifiable blend of bass-heavy menace and jangling that sounds like modernist wind chimes, holds the whole thing together steadily; the film's heightened sound design, similar to the exaggerated sound effects in Haywire but now stressing the noises of offices and apartments rather than fistfights, is used both to heighten disaffection and paranoia, as needed.

All of which is mechanically flawless, and all of which still doesn't answer the question of what the movie is up to with that abrupt shift, or the twist within the shift, or with its surprisingly forthright endorsement of an unpleasantly musty old misogyny straight out of a 1940s film noir. Hell, maybe that's even part of the trick: use 65-year-old generic tropes in a fresh new body, just to see what happens. Not that Side Effects is any kind of noir, but it has their corrupt, nasty soul: every one of the main figures in the film turns out to be pretty unlikable, all of them on their own little power trip in some way or another, and it's simply a great misfortune that the shape this ends up taking is so unconcerned with rather problematic gender overtones.

I can only come back to my initial thought, which is that at a certain point, Side Effects is turning on us, the audience, and very rudely asking a question that nice commercial pictures don't ask: what is our stake in these characters and this plot, and how do we feel about what is done to it? Psychoanalyzing our response to the movie, basically, which isn't out of bounds for a movie about psychiatric medicine, and is well in keeping with a filmmaking career that has, for years now, been obsessed with using the audience against itself. Whatever the case, it's bold and aggressive and completely sure of itself, possessed with a singularity of vision and a fantastic level of craftsmanship to make sure that vision is put across, and I do not like to think that we shall not see the like of it again anytime soon, even when the results are this purposefully inconclusive.