As a filmmaker, Robert Rodriguez is the ebullient one-man band of kinetic pulp. In 1992, he crafted ''El Mariachi'' on a budget of $7,000, and though he has found mainstream success in the years since, notably with the ingenious kiddie tinsel of the ''Spy Kids'' series, he continues to work apart from the Hollywood axis, toiling out of an elaborate home studio in Austin, where he more or less cobbles together his movies from scratch. In a fundamental way, his style hasn't changed. He's still a grandiose primitive of whiz-kid technology.
Rodriguez went south of the border to shoot Once Upon a Time in Mexico, the third entry (after 1995's ''Desperado'') in his ''Mariachi'' trilogy. He photographed the picture himself, using an easy-to-wield high-definition video camera, and he also wrote, produced, directed, scored, and edited (or, as it says in the opening credits, ''chopped'') it. For all I know, he manned the craft-services table as well. Antonio Banderas, coiffed in a handsome, silky-long outlaw style (he should be required to wear his hair this way all the time), is the gunslinger-guitarist hero, who is less a character than a walking spirit, and Johnny Depp is a casually corrupt CIA agent who keeps turning up in assorted disguises. These two provide a hit of marquee wattage, yet ''Once Upon a Time in Mexico,'' in its wildly overwrought, burrito-Western way, is about as close to a home movie as you're likely to see in a megaplex.
Rodriguez, going gonzo at the chopping table, creates some indulgently exciting action scenes. He loves to ride the camera along the ground like a stunt vehicle, snaking up to someone who's holding a very big gun, and then splinter the images like shrapnel. Mostly, though, he careers from one intrigue to the next like a comic-book visionary high on tequila. He has made a dense, jangled, quasi-coherent fantasy of conspiracy and doom, in which everyone on screen is trying to stab everyone else in the back.
In filmmaking, there may be limits, as well as a spark of glory, to doing everything yourself. The joy that Rodriguez takes in each shot is palpable, yet it often feels as if he made up the movie on the spot. Banderas' Mariachi, with his viva-my-dark-eyes smolder of proletarian pride, is out to redeem the honor of Mexico by sabotaging a pair of villains: the fascist General Marquez (Gerardo Vigil), who is about to stage a coup, and Barillo (Willem Dafoe), a drug-cartel boss with an ugly pencil mustache who is only too happy to help him do it. There are subplots involving grisly plastic surgery, the lack of interagency cooperation between the CIA and the FBI, and the Mariachi's long-ago romance with a sultry señorita played by Salma Hayek -- a back story that, for no good reason, doesn't quite line up with the one in ''Desperado.'' Depp, speaking in amused tones of sarcastic indifference, is Sands, the representative of American power who manipulates the action. Depp's performance, with its winking grace notes of Brandoesque flakery (Sands is obsessed with sampling the same pulled-pork dish in every chintzy bar), is as minimal and laid-back as his ''Pirates of the Caribbean'' turn was deep-dish theatrical. Yet here, as well, he evinces a relish for the role of the caustic outsider -- the perfect stance for a born star who won't commit himself to stardom.
The title, with its allusion to the films of Sergio Leone, is, in a sense, a justification for Rodriguez's voluptuous slipshod style. Leone's spaghetti Westerns are now praised as genre masterpieces, yet their tantalizing iconography -- the shoot-outs suspended in time, the looming closeups of gunslingers' faces, the Ennio Morricone music that made it all work -- was always surrounded by a fair amount of lazy and inflated filmmaking. They were epic B movies gone hallucinogenic, and they expressed, and made rapturous, the stoned spirit of the late '60s. ''Once Upon a Time in Mexico,'' too, taps the pulp dynamic of its time, which is zappy, fragmentary action layered on top of too much information. It's pop filmmaking at its headiest, maybe because it never quite gets outside the filmmaker's head.