Brad Pitt finds himself entangled with a mobster who is currently serving time in prison due to a fateful incident where Pitt accidentally ran a red light, resulting in a collision with the mobster's vehicle. When the authorities arrived on the scene, they discovered a body in the trunk—a man who was alive but clearly destined for murder. As a consequence of this mess, the mobster received a five-year prison sentence, and Pitt is now under obligation to work for him to settle the debt.
In the midst of this chaos, Julia Roberts, playing his girlfriend, is growing frustrated with their relationship. She yearns to escape to Vegas, but Pitt holds a heavy burden—he must fulfill one last job for the mobster to ensure his own survival. In an emotional clash, Roberts decides to part ways with Pitt, embarking on her own journey to Vegas while he heads to Mexico for this final task.
Pitt's mission, as directed by the mobster's second-in-command, Bob Balaban, is to retrieve a notorious gun known as “The Mexican.” Upon arriving in Mexico, he successfully acquires the weapon, but fate has other plans. During a local celebration, a stray bullet leads to the untimely death of the man who sold him the gun. In the midst of his confusion, Pitt finds his car stolen, gun included, prompting a frantic chase to recover it.
Meanwhile, on her way to Las Vegas, Roberts encounters danger in a restroom where a gunman attempts to seize her. Just as the situation escalates, she is rescued by James Gandolfini, who claims to be sent by the mobster under the alias Leroy—a sensitive, flamboyant, yet severely unhinged killer tasked with her protection.
Their journey to Vegas takes several unanticipated turns, including a brief romantic interlude for Gandolfini with a gay postal worker. Unfortunately, their joy is short-lived when the gunman returns, leading to a deadly confrontation that sees the postal worker thrown from a balcony. Roberts and Gandolfini manage to dispatch this threat, but dire news reaches them: they must head to Mexico to reunite with Pitt after things go awry.
In Mexico, Pitt faces a cascade of misfortunes, including losing the gun he fought so hard to retrieve. Complications arise when a mob associate sent to eliminate Pitt complicates the situation. Using his wits, Pitt manages to outmaneuver this assassin, but then finds himself in a bind due to a passport error that prevents him from fleeing the country.
In a tense and heartfelt moment, Roberts confronts Pitt, and during a car tire-change mishap, Gandolfini pulls a gun on him. A tumultuous standoff ensues, leading to a surprising and tragic resolution when Pitt realizes Gandolfini's true intentions before fatally shooting him.
As the dust settles, Pitt and Roberts are left grappling with their choices. In a fraught hotel scene, Pitt is once again blindsided when someone attempts to break into his car—an incident he thought resolved. This leads him to a ranch where he confronts the mob boss, [Gene Hackman], who reveals the tragic backstory of “The Mexican.”
The infamous gun was crafted centuries ago as a gift entwined with sorrow and betrayal, amplifying its weight in their current predicament. Meanwhile, Roberts, back at the hotel, finds herself cornered by Balaban, armed and ready, leading to a climactic showdown.
In a twist of fate, Roberts manages to shoot Balaban as he threatens her, marking the vengeance journey in their chaotic lives. The narrative wraps up with Pitt and Roberts symbolically tying the gun’s brass ring as a wedding band, culminating in their car journey filled with bickering and unresolved tension. This rollercoaster ride of loyalty, deceit, and dark humor concludes with an ambiguous yet hopeful outlook for their relationship.
