What if Arthur Pendragon was not a mythical emblem of British chivalry but a street-wise guttersnipe with fast fists and a smart mouth? That’s the premise of “King Arthur: Legend of the Sword,” Guy Ritchie’s attempt to cop the cheeky attitude of a crime flick while maintaining the jowly gravitas of a sword-and-sorcery fantasy. Think John Boorman’s “Excalibur” meets Ritchie’s “Lock, Stock and Two Smoking Barrels.”

If you can’t quite picture it, you’re not alone. Ritchie has tried this kind of genre mash-up before with his “Sherlock Holmes” movies, which achieved a certain level of Victorian snark, but he can’t bring “King Arthur” into focus. Comedic scenes involving foul-mouthed rascals exist side-by-side with operatic (and expensive-looking) set pieces involving wizards, monsters and the Lady of the Lake. Overall, the movie takes itself extremely seriously — except when it tells us not to.

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Charlie Hunnam plays the arrogant if slightly affectless Arthur, whose father, Uther (a stately Eric Bana), was murdered by his power-mad brother, Vortigern (played to the hilt by Jude Law as a kind of 5th-century Nazi in black regalia and iron epaulets). Little Arthur floats in a canoe, Moses-like, to the city of Londinium — one of the film’s few nods to factual history — where the girls at a local brothel take him in.

Arthur’s growth from puny punching bag to bare-knuckle criminal (with a heart of gold, of course) ought to be the best part of this story, but Ritchie zips through it with a quick montage. Too bad, because after that we’re in familiar territory: Arthur pulls Excalibur from the stone, meets a moody wizardress called the Mage (Astrid Bergès-Frisbey) and reluctantly accepts his destiny. Meanwhile, he gathers up the future Knights of the Round Table, whose names range from the recognizable (William, Percival) to the fanciful (Wet Stick, Back Lack). Nods to diversity come from Djimon Hounsou as Bedivere and Tom Wu as a martial artist named George.

Written by Ritchie and others, “King Arthur” is the first in a series of possibly six films. It’s hard to imagine, however, that this schizophrenic combination of saucy wit and bloated effects will last that long.