"This is what happens what you cut the head off a chicken." FBI agent Kate Macer (Emily Blunt) is told this as she is shuttled into Ciudad Juárez, Mexico, by a shadowy group of law-enforcement officials, who soon prove themselves to be just as dangerous and secretive as the ruthless drug kingpins they are hunting. Sicario, the latest movie from rising director Denis Villeneuve, grounds its story in gritty realism as it puts the lead character in the midst of a black-ops task force that's trying to make a big dent in the drug-trafficking underworld. After Macer witnesses something deeply troubling in the film's unforgettable first scene, she accepts a volunteer position on the antidrug force, which is headed by a mysterious authority figure named Matt Graver (Josh Brolin) and his right-hand man, an enigmatic, terrifying enforcer known as Alejandro (Benicio Del Toro). But Macer quickly realizes that she is in over her head, and she is horrified by the methods of enforcement used by Graver, Alejandro, and droll deputy Steve Forsing (Jeffrey Donovan) as they stop at nothing to bring down Juarez's most powerful and ruthless drug lord, Manuel Diaz. She pushes forward with the mission, however, when her boss (Victor Garber in a commanding yet understated role that's completely in his wheelhouse) tells her that she'll need to look the other way when heavy force is used if she wants to make a real difference, adding that "the boundaries have been changed." Even with her faithful, dryly hilarious teammate Reggie (a breakthrough turn for Daniel Kaluuya) by her side to keep her safe, Macer is constantly running head-on into danger, and is forced to keep looking over her shoulder even in those rare moments when she isn't in a life-threatening situation. In addition to being a uniform-wearing target in a foreign land, she begins to question whether her own colleagues can be trusted -- and with good reason. The film culminates with an sequence in which American operatives bust into a tunnel filled with Diaz's employees and contraband, but even that showdown leaves a great deal of unanswered questions regarding the future of the drug war. The sum of Sicario's parts are so good, though, that many viewers will still be left clamoring for a sequel. Villeneuve continues a track record of worthy films with his work here: There isn't a single false move in the visual composition of any scene, and his choice to shoot a number of interactions so that it looks as though a character is speaking directly to the audience provides a big payoff. The director repeatedly stressed that he wanted Macer to be a female action protagonist who felt human and relatable, and the constant, all-too-realistic disrespect and intimidation she receives from her male superiors and counterparts is a recurring theme that will hopefully provoke further conversation. The terrific script was written by actor-turned-writer Taylor Sheridan (whom Sons of Anarchy fans know as Deputy Chief David Hale), and his work is even more impressive when you consider that it's his first produced screenplay. While Sheridan's writing prowess might come as a surprise to some, the haunting music and impeccable cinematography shouldn't. The pulsing, ominous, and horn-laden score comes courtesy of Jóhann Jóhansson, fresh off an Oscar nomination for his original score for 2014's The Theory of Everything. Likewise, cinematographer Roger Deakins is a legend who has earned 12 Oscar nominations, and seems to be forging a partnership with Villeneuve similar to his work with the Coen brothers. The cast is limited to a relatively small ensemble, but the actors are all aces. Brolin is dark and puzzling as Graver, a man whose quirks belie a rugged, potentially dangerous determination. Although his character is given little else to do but provide levity and the occasional wisecrack, Donovan does entertaining work. The only actor the film doesn't properly utilize is Jon Bernthal, who only gets three scenes and whose key moment in the story is fairly implausible. In addition, one character's role is largely undermined by a line of dialogue from an interrogated prisoner, since it prematurely reveals a very important plot point for anyone with a cursory knowledge of the Colombian drug wars. These are minor quibbles, however, for a movie that transcends the crime-thriller genre, delivering real socioeconomic commentary and raising disturbing ethical questions. At the same time, Sicario's simmering pauses provide intense, heart-pounding suspense, and flow smoothly into the scenes of explosive, violent action that arrive when we're least expecting them. Blunt's portrayal of Macer could end up being a model for future crime-drama protagonists; her character is fascinating precisely because she seems like an imperfect person dealing with pain and frustration, rather than a larger-than-life superhero. Meanwhile, Del Toro is utterly menacing, inscrutable, and captivating, and his role as Alejandro can stand alongside his portrayals of Che Guevara (in Steven Soderbergh's Che) and Traffic's Javier Rodriguez as some of his finest work. By the time this movie reaches its climax, viewers will be left with the same emotions as Macer upon arriving in Juarez: shocked by what they have seen, but compelled to sign up for more. Sicario is brutal, mysterious, unflinching, and one of 2015's best films.