Johnny Depp is Hollywood's most valuable player. So far this year, he has turned an average adventure yarn, "Pirates of the Caribbean," into the most enjoyable summer blockbuster of recent years. Now he is the main reason to see the otherwise disappointing "Once Upon a Time in Mexico."

As CIA agent Sands, Depp is slick, sleazy and possibly psychotic. Yet he is always likeable. It's his job to "maintain the balance" in Mexico, whether that means killing chefs whose dishes are too delicious or standing by while the president of Mexico is assassinated.

Depp is the difference

When Depp is onscreen, "Once Upon a Time in Mexico" rocks. When he isn't, which is most of the time, it's uninspired.

Photo © Copyright Columbia Pictures Johnny Depp stars in ''Once Upon a Time in Mexico.''

"Once Upon a Time in Mexico" is the final installment in writer/director Robert Rodriguez's "El Mariachi" trilogy, which began with 1991's low-budget shoot 'em up, "El Mariachi," and continued with 1995's "Desperado," which remains the best of the series.

Technically, the star of "Once Upon a Time" is Antonio Banderas, who plays El Mariachi. (The locals call him "El," as in "The.") He is a drifter who carries a guitar case filled with weapons, which he usually aims at drug dealers and other unsavory types.

Banderas sulks, talks about revenge, plays his guitar and kills a lot of bad guys — and I mean a lot of bad guys. El Mariachi is superhuman. He never misses a shot and never gets hit, no matter how many bullets come his way. The violence in "Once Upon a Time" is intentionally cartoonish, as fans of Rodriguez have come to expect. But that is part of the problem. The action is over the top, but it lacks most of the spark and wit that made "Desperado" so much more fun to watch.

Also, this time the story is darker. Salma Hayek, Banderas' love interest in "Desperado," gets second billing ahead of Depp, but she appears only in flashbacks, her character having been murdered by a jealous former lover.

That former lover is a rogue general who wants to kill the president. So, Sands recruits El Mariachi to eliminate the general and his backer, a drug kingpin played by Willem Dafoe. But the endless betrayals, reversals and plot twists that follow happen so quickly that none makes an impression. The film could have used another 20 minutes to allow the plot to advance at a less hectic pace.

More time might also have allowed Banderas to do something other than glare, kill people and be overshadowed by his co-star. Or maybe that's enough. After all, Russell Crowe won an Oscar for less.