What is a movie? That's a question Martin Scorsese, as great a director as he is, really can't answer clearly in "Gangs of New York." Such confusion is at the middle of this muddle.

For if a movie is a time machine, a trip to another epoch, a realization of looks and sounds and sights not seen in 140 years, then "Gangs of New York" is a great movie.

It has the genius of thereness to it. You are there when two tough crews named the Dead Rabbits and the Natives meet at Five Points, a raggedy intersection of streets in Lower Manhattan in the 1840s, back when Gotham was essentially a slum built on a cesspool in a swamp. You are there as these two deadly armies clash with flail and blade and club. You are there when they fight in top hats and spats and sashes and mustaches that curl up like the tips of a caliph's slippers, and the battle looks more like Culloden -- Scots vs. English, 1746 -- than anything else.

And you are there when the Natives -- long-settled old stock who've forgotten their own immigrant origins -- vanquish, then banish the Dead Rabbits, the Irish. You are there when folks think the name "Dead Rabbits" is cool and dangerous.

So if the idea expressed by the word "movie" is some sort of meta-museum of history that shows you exhibits, then "Gangs of New York" cannot be missed. Scope, vision, power, immaculate filmcraft, sense of wondrous newness, sense of history's melancholy vapors, sense of the bloody ground that is this country of ours. Friend, is that your idea of "movie"? Then be my guest, sit back, enjoy, learn, stretch, expand, be morally improved.

Unfortunately, if your idea of movie comprises one word and that word is "story," you're going to be disappointed. For under its scope and reach and passion, "Gangs of New York" is pretty ordinary stuff. You've seen it before at least a hundred times, and it's extremely dispiriting that a director of Scorsese's power and experience didn't choose something more complex.

It's as if he preferred to concentrate on the production -- the building of the sets, the sweeping moves of the cameras, the depth and detail of the compositions, the montage of furious battle action -- rather than on the dramatic issues and, oh yeah, taking up the rear, the human beings who live them. It's just the old revenge melodrama, the one about the son seeking payback for the murder of his father. When, after exile, he returns to the arena, he is so gifted that his father's murderer, a powerful man by virtue of that murder, is attracted to him, and invites him into the gang. So Our Hero is tempted: success or vengeance? Wouldn't be a movie if he chose success, right? You already know that, so you already know everything.

It begins with that street battle, but the point of view is that of the young Amsterdam Vallon's. His father, Priest Vallon (Liam Neeson), leads his Irish boys against the nativist Natives -- a mob of guys terrified then (as now) by the new waves of immigrants, who they know must be kept down or they and their little private world will be overwhelmed.

The fight, at Minute 3 and lasting through Minute 9, is pretty much the high point of the movie. Scorsese has always had a genius for depicting violence ("Taxi Driver," "Raging Bull") and here he really outdoes himself, with an evocation of the racing fear and exhilaration of close-quarters combat that leaves you shaken. But some of the blood in the snow comes from Priest, who is defeated by the Native leader, a fiery galoot named Bill the Butcher (played with screaming bravado by Daniel Day-Lewis).

So the boy Amsterdam watches his father die, particularly the coup de grace issued by Bill to put the writhing man out of his pain. But the plucky Amsterdam steals that knife and runs off with it, and the camera stays with him as he penetrates the multi-tiered Dead Rabbits' command structure (which I never quite understood) and manages to bury the knife, which he will recover when he turns into Leonardo DiCaprio and then . . .

And then nothing. That's a suggestion of the story's lack of narrative savvy: The whole first act of the movie is about the theft and hiding of that knife, and its recovery 16 years later, in 1863, in a move heavily freighted with symbolic foreshadowing. All from Storytelling 101. And then the knife just disappears. So what was the point of dramatizing this knife above all others in a universe of knives? That question is left hanging.

Continually, Scorsese's impulse to inform overwhelms his instinct to dramatize. He is so proud of his mastery of the source material, a pulpy, anecdotal true-crime volume published at the turn of the last century. A whole sequence, for example, involves the beautiful Jenny Everdeane (Cameron Diaz), professionally a Five Points pickpocket, who goes on an uptown foray to crash mansions in a maid's outfit and steal the places blind. This, we are informed, is so specific a crime, its practitioner has a colorful old-timey name: she's a "turtledove." But so what? Everdeane as turtledove never again figures into the plot; she just becomes, generically, "the girl" in a predictable triangle with Snidely Whiplash -- Bill the Butcher, that is -- and Amsterdam.

As for DiCaprio, he's certainly the weakest performer up there. He's cast as a deadly, beautiful street youth, with a quick cunning and a will to do battle and seek revenge. Well, you believe the beautiful part. But somehow he never projects the city-rat toughness of a young slum champ. Remember Garfield fighting his way out of the mean city back in "Body and Soul"? No, you don't? Well, then try this: DiCaprio is never feral and edgy like, say, the young Robert De Niro in Scorsese's "Mean Streets" so many years ago. Still a blank? All right, let's go here: He's never as tough and believable a city kid on the hustle as Eminem is in "8 Mile." He's way too uptown. He should be wearing a black turtleneck and Italian director glasses and Prada shoes.

A whole middle hour (of three) follows plot twitches of little consequence, such as Amsterdam's friendship with another street kid grown too handsome (Henry Thomas), the interplay between Bill the Butcher and Boss Tweed (Jim Broadbent) and the most important of all, will the Dead Rabbits' hallowed symbol of identity (yes, that would be a dead rabbit) be hoisted downtown again. The facts are fascinating: Are you aware that firemen in the 1860s were essentially gang-controlled looters whose main job was to steal the valuables of the house before it turned to ash? Interesting, but it has nothing to do with the story.

But it finally seems that the movie itself has nothing to do with the story: Scorsese pretty much forgets about it in the end. It's clear he wanted to say something about the New York draft riots of 1863, an ancient holocaust that has all but vanished from memory as it was so overshadowed by contemporary slaughters at Antietam and Gettysburg. He just has nothing to say about it.

It's a shameful episode, a spasm of urban bloodletting in which, among other things, more African Americans were slain than in any other single event in American history. Yet Scorsese's treatment of it is distressing. In the first place, it blows into town like a hurricane, presented almost as a natural occurrence, and it all but obliterates the nominal climax toward which the film has clearly been building, the return match between Squashed Bunnies and Native natives; even the last meeting of the Butcher and Amsterdam is muted, as if obscured in a fog or a squall.

In the second place, it has an extremely curious and debilitating effect: It takes the New York draft riots completely out of a moral context. The movie fails to make the point that even a little research convinces you of -- that these folks, who were refusing to go to war to end slavery and save the union, were hardly victims; they were essentially in open, murderous rebellion against, er, America and were committing violent treason. And that's before they started lynching black people. Considered in that light, the arrival of the Army hardly feels like the neutral tragedy of Scorsese's imagination. You want to rape, burn, pillage and tear down, and murder any black person you can get your hands on? Then say hello to the Fifth Pennsylvania Infantry with its bayonets fixed and fingers on the trigger. Then as now, there are always consequences.

Gangs of New York (165 minutes, at area theaters) is rated R for extreme carnage.

Leonardo DiCaprio, center, fails to project the city-rat toughness of a young slum champ avenging his father's death.Cameron Diaz and Daniel Day-Lewis are two sides of a predictable triangle in Martin Scorsese's epic.