There is this difinitive moment about halfway through "Snake Eyes" in which the movie suddenly and completely collapeses without warning.

I've never seen anything like it. It's not that you look back after the credits roll and realize where the movie went wrong. It happens right before your eyes.

The film takes place entirely within a casino-arena in Atlantic City, where a politician is assassinated during a boxing match. Nicolas Cage stars a connected and crooked, wild-eyed cop who orders panicked fight fans sealed inside the venue when he suspects a conspiracy, and a relentless search begins for members of the plot.

Director Brian DePlama reveals the story like a high-stakes blackjack dealer at the casino's tables, dictating the pace and tension of the movie by patiently revealing the plot one card at a time.

The first 40 minutes of "Snake Eyes" is thick with gripping pandemonium. The murder goes down at the end of a masterful opening tracking shot that follows Cage for 10 minutes, unedited and uninterrupted, as he plays greasy-cool hot shot to bookies, crooks and fight promoters, then eventually takes a ring-side seat next to the target, the U.S. Secretary of Defense.

With this energetic set-up followed by a stunning murder, the rest of the film should become a tense whodunnit as soon as Cage orders the arena closed.

But after only 40 minutes, DePalma's dealer skills buckle and he spills all his cards.

In the middle of a scene -- in the middle of a conversation, no less -- the conspiracy at the center of the plot is revealed in its entirity and suddenly the movie has no place left to go. It disintegrates like a building being dynamited. The story is over, but half the movie is still in the projector.

While it lasts, the emerging conspiracy is potent stuff -- disorienting, hurried and complex. There's a pair of mysterious femme fatales who dissappear into the stampeding crowd when the shots ring out. There's a known terrorist to initially take the fall. There's a motive unveiled concerning doctored missle tests. And there's Gary Sinise, playing the head of the Secretary's security detail and also a friend of Cage's crooked cop, who is not what he seems.

Together Cage and Sinise begin to investigate and while questioning key witnesses the assassination is reviewed from several characters' points of view. Cage pours over survailance tapes and broadcast footage from the fight, in which he discovers the champ took a dive (it's insultingly obvious), and it becomes clear something sinister is afoot. Now he doesn't know who to trust.

But for everything DePalma does right in the stylish and inventive first half, there are twice as many mistakes after that fateful dead-end scene.

With nothing to do for the for the last several reels, DePalma plies the picture with gimmicky hide-and-seek scenes. After createing a crisis of conscience for Cage to play with, his character becomes suddenly virtuous when the baddies try to buy his silence.

The ugency is lost, despite the fact that our anti-hero finds himself on the run from the assassins, and DePalma's tendency to assume you are paying as much attention as he is (remember Tom Cruise's confusing revelation near the end of "Mission: Impossible"?) causes several flash developments to elicit a collective "huh?" from the audience.

It's understandable that Cage and Sinise could be drawn to this material. "Snake Eyes" had the poetnetial to be extremely slick and clever. But the movie's flat second half rubs off on these two normally exemplary actors. Cage's character completely and inexplicably changes personality, becoming souly interested in Truth. Sinese sleepwalks through his role as if he'd lost interest somwhere in the middle.

Like many Brian DePalma movies, "Snake Eyes" harks somewhat of updated Alfred Hitchcock as seen through the eyes of a B-movie hack. He comes close to making a good movie here, but once he's prematurely blown the plot, the whole atmosphere goes limp and try as he might, he can't get it up again.