In a legendary, dark dusty bar, a challenge is made by the fearsome general at the end of the room. A man rises from his chair to meet it, reaches under the table, pulls out a guitar and begins to play. This mariachi plays faster and faster as the general looks at him in shock and then, with a wink, he cocks the guitar and opens fire. And just when it seemingly can’t get any better, Salma Hayek enters.

I would like to report that all of “Once Upon a Time in Mexico” is as good as this opening sequence, but I can’t. It quickly uses up all its charm in the first fifteen minutes and by the time Johnny Depp is uttering lines such as “Are you a Mexican or Mexicant?” the mythic proportions promised by the beginning are left lying in the dust.

“Mexico” again brings us more of the saga of El Mariachi (Antonio Banderas), the hero of director Robert Rodriguez’s earlier film “Desperado,” who is again out for revenge. This time it is a young, ruthless CIA agent named Sands, played by Depp, who brings him back into the game. Sands hires El Mariachi to stop a military coup of Mexico led by General Marquez who, coincidentally, killed El’s wife Carolina (Salma Hayek).

From his very entrance Agent Sands, never seems to make much sense, interrupting his lunch companion’s (Cheech Marin in a cameo) epic tale to offer him a taste of his food. I suppose that Sands is intended as the American character who doesn’t understand the heart and spirit of Mexico, but Rodriguez, who also wrote and scored “Once Upon a Time in Mexico,” never really completes Sands’ character. Instead we are left with a hazy poker face who shoots a cook for fun, sits around eating pork all day, and gives the Skywalkerian job description of “restoring the balance.” This lack of depth leaves Depp with nowhere to go but to his past, so he gives us an amalgam of other performances from better films. At one point he even finishes a line with a slightly drunk, slightly British “Savvy?” and his brilliant turn as Jack Sparrow in “Pirates of the Caribbean” comes longingly to mind.

Through a series of fast-paced flashbacks it becomes quite clear that El Mariachi is doomed to one dimensionality — he spends the movie avenging Hayek’s death, who is only in the film for about five minutes. This is rather unfortunate, for Hayek is clearly Rodriguez’s muse (a role she perfected in “Dogma”) as those five minutes are by far the best in the movie. Rodriguez is not as kind to his other actors. One glaring case is his uninspired use of Willem Dafoe as the cartel boss who hires General Marquez to pull off the assassination of the Mexican president. Dafoe has incredible range as an actor, from his performance as the ultimate enigmatic hero, Jesus in “The Last Temptation of Christ” to Nosferatu himself in “Shadow of a Vampire.” Yet what does Rodriguez give him? Stand around with a malicious grin and kill someone every few minutes — oh, and look Mexican.

But who cares? This movie isn’t about the plot or acting, it’s about action. And Rodriguez makes good with some fairly tried-and-true staples of the genre. The standard “random guy shot off roof” makes an appearance as does the “sledding on creative object down the stairs” number and we mustn’t forget the “gasoline tanker truck comes out of nowhere to hit the speeding bus” bit.

Luckily the movie is not entirely made up of these cliches; Rodriguez contributes some genuinely new action sequences. There is a chase scene through a cactus field, a remote-controlled exploding guitar case, and an incredible swinging descent down the side of a building. If the movie were entirely made up of tongue-in-cheek innovations such as these, it would be a lot more fun — but no, there had to be dialogue. My favorite line was uttered by a compadre of El Mariachi in a very solemn tone: “A man who wants nothing is invincible.” I bet they didn’t think anyone would examine that one up close. Seemingly profound, yet totally devoid of meaning. And that sums up “Mexico” in general. Rodriguez spent so much time appearing hip and ironic that he forgot the substance or even most of the fun.

Case in point: the score. A tango is a beautiful thing, blending music and dance to create seduction. The movie begins with El Mariachi playing a tango on his guitar. This simple scoring quickens the blood, but Rodriguez quickly abandons guitar music altogether to lean on a heavy Hollywood score. He returns to the guitar at the end, but by then it is too late. For as any good musician knows, you can’t stick a polka in the middle of a tango and expect your audience not to notice it.

PS: Stay for the credit music, with Salma Hayek revealing a surprisingly good voice — it far outmatches the score.