“Gangs of New York” starts out an epic and ends up an epic mess.

Director Martin Scorsese’s intricate portrait of a buried, bygone era is often riveting. The set design, costumes, weaponry and general aura of the project represent Hollywood skill at its finest.

The film is also a narrative free-for-all that is unevenly acted and borderline preposterous. What begins as a highly entertaining but very insular struggle between two feuding men shifts into a large-scale history lesson about the 1863 draft riots.

Emblematic of this division between what works and what fails is Daniel Day-Lewis. Scorsese coaxed the Oscar-winning Brit out of career slumber to portray Bill “The Butcher” Cutter. This knife-adroit dandy comes across as stunningly believable in some scenes and downright vaudevillian in others. Day-Lewis is certainly not lacking energy; at times the actor is so over-the-top he seems to be floating above the set.

In 1846, Bill is the leader of an Anglo-Saxon “nativist” gang who is bent on controlling the Five Points of lower Manhattan. (“It wasn’t a city,” the narration informs. “But a furnace where the city some day would be forged.”) During a bloody battle with the forces of Priest Vallon (Liam Neeson), who has united the immigrant Irish, Bill manages to slay the cleric and seize command.

Vallon’s son Amsterdam (Leonardo DiCaprio), who witnessed the combat, returns to the region years later to exact revenge on The Butcher. Initially, Bill develops a liking for Amsterdam and shepherds him into the complex, corrupt world of the Five Points. As Amsterdam learns, “It’s a funny feeling being taken under the wing of a dragon: It’s warmer than you think.”

But before long, the men are on opposite sides of a bloody turf war.

As in many of Scorsese’s pictures, the New York director is most engaging when putting his visual/editing flair into explaining how some system works. Whether it’s hotel security surveillance in “Casino” or the game of 9-ball in “The Color of Money,” he somehow transcends the activity as soon as he starts dissecting it onscreen.

Daniel Day-Lewis stars as a leader of nativist tribes fighting for dominance in Martin Scorsese's Gangs

Likewise, “Gangs” is fascinating in its detail of street crime and the political machinations that fueled it within this near-Dickensian environment. The viewer learns that fire brigades were often little more than rival gangs, and responding to a burning building was merely a sanctioned way for hostile precincts to rumble.

Also scrutinized are the various guilds of urban criminals, highlighted by tailing a bludgeon (a female pickpocket, in this case played by Cameron Diaz) while she goes about her trade. Her victims — all male, of course — are charmed by her flirtations until they realize she’s methodically pilfered their possessions.

Yet Scorsese falls victim to his own cinematic misdeeds.

For starters, there’s the casting of DiCaprio. He’s the nominal hero, but Day-Lewis is so much more intriguing (albeit cartoonish) that he makes the “Titanic” heartthrob seem like a dud. (Look to DiCaprio in next week’s “Catch Me If You Can” for a performance that far better suits his canny talents.)

Next, Scorsese provides no reason that Amsterdam is allowed to live once Bill discovers his motivations. The Butcher is depicted as among the most ruthless gents of his era, and yet his punishment to Amsterdam for an attempted assassination is nothing more than a symbolic bitch-slap. The only reason this young man’s limbs aren’t repossessed is in order for him to fulfill the tired conventions of the justice-will-be-done plot.

There’s also very little motivation for how or why the charisma-free Amsterdam is able to recruit so many people so quickly to try and overthrow the deadly Bill. One minute these hordes are embracing the system that’s been in place since the nativists gained power 17 years earlier, and the next they’re willing to flock to Amsterdam like he’s the pied piper.

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What’s in it for them? Will anything really change if he wins?

Ultimately, what knocks the ambition of this lofty movie down to street level is the misguided conclusion. Scorsese becomes so enamored with trying to mount the full spectacle of the New York Civil War draft riots that he virtually abandons his two lead characters. The dramatic non-showdown between their respective gangs echoes that of “Monty Python and the Holy Grail” — a finale where the comedic filmmakers tried to deliver the most foolish, frustrating and anticlimactic resolution they could conceive.

They succeeded in that goal … and so does Scorsese.