In a small Mexican town, a ruthless criminal, nicknamed Azul, breaks out of jail and vows revenge on the local drug lord, Moco, who put him there in the first place, by using a guitar case which carries a small arsenal of guns. At the same time, a young mariachi arrives in the town looking for work, carrying his guitar case with his signature guitar. From the confines of his heavily guarded villa on the outskirts of town, Moco sends a large group of hitmen to kill Azul, but because both men are dressed in black and carrying guitar cases, the hitmen mistake the mariachi for the criminal. Only Moco knows what Azul looks like. The mariachi kills four of Moco's men in self-defense.
As the mariachi seeks refuge in a bar owned by a woman, named Dominˇ, he falls in love with her. Unfortunately, her bar is financed by Moco. When Azul visits the bar for a beer and information about Moco, he accidentally leaves with the mariachi's guitar case. Moco's thugs capture Azul on the street but let him go when they learn that the case he is carrying contains only a guitar. A short time later, the mariachi is captured and taken to Moco, who identifies him as the wrong man and sets him free.
Meanwhile, Azul, who has no directions to Moco's home, takes Dominˇ with him and orders her to take him to Moco's, or he will kill the mariachi. Dominˇ agrees in order to save the mariachi's life. When they arrive at Moco's gated compound, Azul pretends to take Dominˇ hostage in order to gain entry. Moco soon realizes that Dominˇ has fallen for the mariachi and, in a rage, shoots both her and Azul. Suddenly, the mariachi arrives to find the woman he loves gunned down. Moco then shoots the mariachi's left hand, rendering him useless as a guitar player.
However, overcome with grief and rage, the mariachi picks up Azul's gun and kills Moco, taking revenge for Dominˇ's death. Moco's surviving henchmen, seeing their leader dead, walk off and leave Moco's body and the wounded mariachi behind. In the final scene, the mariachi leaves the town on Dominˇ's motorbike, taking her dog and her letter-opener to remember her by. His dreams to become a mariachi have been shattered, and his only protection for his future are the weapons of Azul, which he took along in the guitar case..