NOTE: This spoiler was submitted by Jeremy
We first see some text that explains that the origin of the word "sicario" comes from the Roman zealots that occupied Palestine. In Mexico, the word means "hitman".
Chandler, Arizona - A SWAT team moves in toward a house where they suspect there to be hostages held by cartel leader Manuel Diaz (Bernardo P. Sacarino). Kate Macer (Emily Blunt) and her partner Reggie Wayne (Daniel Kaluyya) move in with their rescue team and burst in through the house with the van. They capture a few suspects, while Kate kills a man that tried to shoot her. The team then finds dozens of bodies stuffed into the walls with bags over their heads. Most of the team goes outside to puke. Kate's superior Dave Jennings (Victor Garber) calls her inside. Outside, several officers notice something from a nearby shed. A bomb goes off, killing two officers.
At the station, Kate is brought in to meet Matt Graver (Josh Brolin). He is brought in to help catch Diaz, along with a mysterious agent named Alejandro (Benicio Del Toro). Kate is asked to join the team, which she only agrees to so she can catch the men responsible for the deaths she witnessed that day.
Kate initially believes the team is going to El Paso, but they end up going to Juarez, Mexico. Once there, the team rolls with a whole squad of cops led by Steve Forsing (Jeffrey Donovan) through the city. They drive up to a crowded parkway. Alejandro spots two cars with gang members and he notices they have itchy trigger fingers. The cops slowly move out of their vehicles and approach the gang members. One of them gets out of their car slowly as well, gun in hand. The officers tell him to lower his weapon, but he reacts quickly, getting himself and his buddies shot to hell. The other gang members suffer the same fate when one of them tries to draw his weapon on the cops.
Kate argues with Matt on the actions taken on the parkway, as well as the whole way that they are leading this mission. The stress has even caused her to take up smoking again.
Elsewhere in Mexico, we meet a man named Silvio (Maximiliano Hernandez). His son tells him that he has a soccer match later that day. Silvio goes through his daily routine before heading off to work. We see him wearing a police uniform as he walks to his car.
The team stops a bunch of buses filled with immigrants. Alejandro questions some of the men through a sort of process of elimination to determine which of them could work for gangs and which of them could have ties to Diaz. In their search, the team finds bags of money to be laundered and wrapped in blue-pink wristbands.
Some of the team goes out for drinks later that night. Reggie introduces Kate to his buddy Ted (Jon Bernthal). After a few drinks, Kate goes back to Ted's place to hook up, until she notices he took out from his pocket a wristband similar to what they found earlier. Kate tries to get away, but a struggle ensues as she tries to reach for her gun. Ted starts to strangle Kate until Alejandro walks in and points his gun in Ted's face, making him release his grip on Kate.
Ted is taken into custody. Alejandro enters the car with him and beats his face until Ted tells him what he can say regarding his business with Diaz.
Kate eventually learns that she and Reggie were just needed to extract drugs from the case, essentially just making them pawns in the grand scheme of things for the team.
The team succeeds in taking away a substantial amount of money from Diaz's account, forcing him to make his return to Mexico. Following that, the team moves in on a raid into one of Diaz's warehouses. There, Alejandro finds Silvio and holds him at gunpoint. Kate aims her gun at Alejandro, but he shoots her in her vest and tells her to never do that again. He takes Silvio and makes him drive away. Kate returns to Matt and learns that Alejandro is a hired gun by the CIA to catch Diaz's boss, Fausto Alarcon (Julio Cedillo), because he decapitated Alejandro's wife and threw their daughter into a vat of acid.
Alejandro forces Silvio to drive until they see Diaz's car. Alejandro makes Silvio pull Diaz over and force him to step out of the car. Silvio walks out to confront Diaz, only for Alejandro to shoot and kill Silvio and shoot Diaz in the leg. He walks out to kill Diaz at a closer range and then take his car.
Alejandro arrives at Fausto's home and kills all of his guards. He walks outside to find Fausto having dinner with his wife and two sons. He aims his gun at Fausto and tells the wife and boys to continue eating and to stay calm. Fausto tells Alejandro that his wife wouldn't like what he's become, a lawyer-turned-hitman. Alejandro brings up his daughter. Fausto says it was nothing personal, but it was to Alejandro. He says in Spanish, "Time to meet God". He shoots the wife, then the two boys dead, to Fausto's horror. Alejandro tells him to finish his meal, moments before he shoots Fausto dead.
Kate finds Alejandro in her apartment. He hands her a contract for her to sign to confirm that everything they did was by-the-books. Kate refuses to sign it. Alejandro holds a gun under her chin, stating she'd be committing suicide if she doesn't sign. He wipes tears away from her cheeks while never removing the gun. Kate reluctantly goes ahead and signs. Alejandro leaves. Kate grabs her gun and aims it at him from her window. He turns around and waits for her to pull the trigger, but she just slowly lowers it and lets him walk away.
Silvio's son stands over his father's empty bedside, almost as if he knows his father won't be coming home. His mother takes him to his soccer match. During the game, everybody hears gunshots in the distance. They quickly proceed to carry on with the game.