If a body wanted to be unnecessarily reductive in every possible way, one could say something like: Sicario is the Mexican drug cartel version of Zero Dark Thirty, with Emily Blunt replacing Jessica Chastain as the woman driven to the breaking point in competing with violent male nihilism. I say, let's not be that reductive. But let's concede that if necessary, we could be.

Blunt plays Kate Macer, an FBI agent we first meet leading a raid in Arizona against a house of suspected cartel-linked kidnappers. Her performance on this (and, presumably, earlier) missions puts her at the front of the line, when the CIA, in the form of Matt Graver (Josh Brolin), comes looking for FBI assistance in forming a multinational sting to find and capture Manuel DΓ­az, a major figure in the cartels. And from here, we can talk about the film in one of two ways. It is, on the one hand, a procedural about the U.S. government's black ops efforts in Mexico, one that ponders with grave horror the excesses and flatly illegal shenanigans perpetrated by Matt and his Mexican partner, Alejeandro Gillick (Benicio Del Toro), viewing them through the increasingly nauseated and righteously angry eyes of Kate. It is, on the other hand, the closely-related story of a series of moral quandaries lived by Kate, as she grows thus nauseated.

This is the main place that the Zero Dark Thirty analogy holds water. Like that film, Sicario perpetually feels like it should be read predominately through a lens of political commentary (the key difference: ZDT was noticeably allergic to real-world politics, while Taylor Sheridan's script for Sicario seems quite convinced that it is first and above all a political document); like that film, it's a great deal more interesting as a character study. Kate is not, in fairness, an all-time great example of writing; it's essential to Sicario's basic function that she is almost exclusively a reactive character, forced to witness depraved acts and grow despondent at her inability to stave them off. And this is true of the depravities committed by the cartels as much as by the amoral government officials attempting to combat the cartels using nearly equivalent violence - the film's opening scene sets the tone nicely enough, with Kate and her partner Reggie Wayne (Daniel Kaluuya) discovering a cache of rotten plastic-wrapped corpses in the walls of a house, with Blunt's face washing out with a neatly-controlled look of disgust and sadness (Reggie, for his part, drifts in and out of the rest of the film as a sort of Chorus; it's a bit awkward, frankly, but it helps the movie immensely to have one other human being who isn't some breed of psychopath or sociopath).

Granting that Kate is a largely inert figure in the grand scope of things, she's still a magnificent protagonist, benefiting greatly from a perfect performance by Blunt, who keeps winding herself tighter and tighter along the course of the film's two hours. In the film's tensest moments, her whole body practically quivers; at other points, she collapses like a rag doll, frequently using cigarettes like an anchor to keep herself upright (Sicario's use of cigarettes as an organising motif is fascinatingly dated, but note-perfect; it's a keen reminder that, deadly though they be, there's nothing that can compare for giving actors interesting pieces of business with their hands and face). Playing an arc with almost no action to drive it, Blunt is obliged to expressively inhabit a series of subtly-shaded variations on just a few emotional states, and she does so extraordinarily well, providing a deeply compelling character in her own right as well as serving as the moral bellwether for Sicario's plunge into wretchedness. Brolin and Del Toro both give sturdy supporting roles - after a second viewing, much more deft turns than I'd credited either one of them with at first - but they're shadows next to Blunt.

Outside of her dominating performance, Sicario, under the rock-solid control of director Denis Villeneuve, trots out a host of finely-tuned formal elements to augment the basic thrust of looming, onrushing chaos that Blunt responds to so well. It is, truly, one of 2015's best top-to-bottom exercises in film craftsmanship. Joe Walker's editing is full of little pounces: moments where the action stretches and stretches in weirdly slowed-down moments until a spattering of quick cuts drives us through their climaxes (one particularly great moment comes near the end, a dinner scene that's absolutely one of the year's most perfectly tense moments). Roger Deakins's cinematography is unsurprisingly top-notch; it's less bold in its exploration of the particular textures of digital cinematography than his work on Skyfall or Villeneuve's Prisoners, and it's certainly true that the central conceit of the images - making the Mexican and Arizonan landscape and sky wash out to an equally dead shade of white - is neither terribly innovative, nor any real test of Deakins's skill. But it's still flawlessly executed, and when the action shifts to night, there's a shining, sharp chilliness to the imagery that's both beautiful and menacing.

The film's real strength, though, lies in its use of sound. The mix is full of muffled noises that surround the characters and drift out into the distance; the vastness of the empty, dusty exteriors is total and crushing, and the interiors (especially the FBI) are stifling. Even better is JΓ³hann JΓ³hannsson's score, if we can call it that; the music here is mindlessly atonal, dominated by a bassline throbbing from the opening (a black screen with droning notes) right to the penultimate scene. Honestly, for all that the whole film is smartly put together, the music and Blunt's performance are all it really needs: the audio causes a blanket of unsettling dread, the lead actor responds and amplifies that dread.

Now, having said all of that, Sicario suffers from one pretty severe shortcoming: it it is merciless. There's very little hope to any of this, and virtually nothing to draw from it: awful, violent people are awful and violent, and good people can (and should) be horrified by this, but there's no meaningful way to stop it. Happily, the film cuts off at a reasonable enough length (and to me, it feels shorter than it is) that it's not abject, but it's still basically suffocating and depressing. Immaculately made, and boasting one of 2015's greatest performances, so by all means I am pleased to have suffered through it; but there is, ultimately, a limit to how artistically meaningful something so fixated on agony can possibly be.