Robert Rodriguez wanted to make an epic Western set in Mexico to complete his “El Mariachi” trilogy. And on one level, he has succeeded: the picture is as sweeping and full of bravado as a Sam Peckinpah or Sergio Leone Western. But Rodriguez erred by sidelining the main attraction of the first two installments: El Mariachi himself, the iconic and deeply sensual hero who displays an unwavering, if extralegal, sense of justice. Without him in the lead, the film is as flat as the hot Mexican desert where it’s set. In his stead, Rodriguez has cooked up a new set of comic-book characters who bluster and shoot their way through a convoluted and violent tale. And though he edits some of the shootouts and chase scenes with considerable wit and energy, the plot itself will leave even the most attentive viewer thoroughly confused.

Set in a scenic, dusty town in the middle of nowhere, “Once Upon a Time in Mexico” picks up a few years after “Desperado” ended: with El Mariachi living in isolation following the murder of his wife, Carolina (Salma Hayek), and their little daughter. (The swashbuckling opener, it turns out, is a newly contrived flashback to tie the two movies together.) A rogue CIA agent named Sands (Johnny Depp) shows up in town and hires the gunslinging El Mariachi to thwart a plot by drug kingpin Barillo (Willem Dafoe) and an ally to overthrow the president. El Mariachi has his own reason to murder Barillo–revenge for Carolina’s murder–and enlists his low-key mariachi-cum-sharpshooter sidekicks (Enrique Iglesias and Marco Leonardi) to help. But Barillo knows of the mark and has murderous designs of his own. Before long, Sands–who, despite having some of the film’s best lines, remains too lackluster to serve as its centerpiece–is playing everyone off everyone else until the double-crossed are triple-crossed, we think, and the guns are all cocked.

The showdown takes place, naturally, during the country’s Day of the Dead celebration, when the streets are already creepily full of marauders in skeleton masks. Ultimately, scores–maybe hundreds–of Mexicans are sliced and diced and slugged and shot in a matter of minutes. Who knows whose side they’re on, and who cares? Rodriguez may have wanted to create a sprawling, significant epic that would stand the test of time, but what he ended up with is yet another action flick geared toward teenage boys. El Mariachi may have avenged the girl, but he couldn’t save the movie.