“A Knight’s Tale” offers a glaring example of how Hollywood cinema has been dumbed down over the past four decades. The central joke of writer-director Brian Helgeland’s picture, which is being advertised as a sort of “Braveheart” epic but is actually a romantic comedy with action elements, is based on the insertion of anachronistically modern elements into a medieval setting–in this case, the fourteenth-century era of the Hundred Years’ War. (You could say that the picture’s a sort of mirror image of “Just Visiting”–medieval knights aren’t transported into the modern world; modern attitudes are instead exported to theirs.) The same conceit was at the heart of Anthony Harvey’s 1968 classic, “The Lion in Winter.” In that case, however, it was used to draw a sly, sophisticated portrait of the back-stabbing Plantagenet family headed by English King Henry II in the 1180s; superbly cast and acted, the picture was a cross between one of Shakespeare’s historical plays (whose high-brow language it even mimicked) and “The Little Foxes,” or (if you prefer) a bitingly funny (and sometimes sad) account of the friction within a medieval version of the Ewing clan a full decade before “Dallas” hit the airwaves. It was a picture for grownups.

“A Knight’s Tale” employs similar devices, but pitches them much lower, ending up as a far broader effort which panders shamelessly to today’s dominant youth market; it’s aimed not at grownups, but at persons of adolescent tastes, whatever their physical age. It’s the tale of a young indentured servant to a knight on the jousting field, who (feigning the requisite nobility) takes up the lance himself when his master dies, soon becoming a superstar on the “circuit.” He also falls for a beautiful damsel whose hand is being sought by his chief rival, a sneering, arrogant count. The models here are hardly on the same level as those that James Goldman had in mind for “The Lion in Winter.” The action element, involving young William Thatcher’s desire to excel in the jousts, is a bit reminiscent of the 1954 “Prince Valiant” (with Robert Wagner sporting that hilarious pageboy wig), but it owes even more to the “Rocky” formula that has plagued us since the mid-seventies; it merely exchanges boxing-gloves for lances. And the romantic subplot, with the shy, socially inept lad trying to overcome the obvious advantages held by wicked Count Adhemar to win the hand of Lady Jocelyn, is little more than the triangle that’s so familiar from countless high-school flicks; only the clothes (and ages) of the participants are different. (Curiously enough, Heath Ledger, playing Thatcher, is here the slightly dorky dude who eventually gets the girl–quite a reversal from his turn in “10 Things I Hate About You,” in which he played the hunk instead.)

In jazzing up these two formulas with jokey anachronisms, moreover, Helgeland has resorted to the most obvious sort of riffs. The big joke about the jousting tournaments, for instance, is that they’re set up like modern sporting extravaganzas, especially pro wrestling matches or football games. Each event literally begins to the sound of “We Will Rock You,” and the spectators often break into the “wave” or pump their arms with a resounding “Yes!” when their favorite scores a point. Every participant is even given a promoter who announces his man to the crowd in Vince McMahon fashion, and the one whom William picks up is none other than the young Geoffrey Chaucer, presented as an inveterate gambler and dazzling pitchman. (This seems a pretty desperate theft from “Shakespeare in Love,” but here the depiction of the author isn’t related in any way to his work and so just seems an easy and pointless gag. Perhaps it was merely the fact that some high-school curricula still include “The Canterbury Tales” that led Helgeland to insert Chaucer into the script as a name some younger viewers might actually have encountered.) As to the romantic side of the flick, there’s absolutely nothing new. Thatcher engages in the same sort of fumbling schtick that’s so familiar from John Hughes movies and their imitators–a scene where he’s browbeaten by Jocelyn in a church (naturally she’s the modern sort of independent gal who’s not about to fall for a guy easily and will set up a test whereby he must prove the depth of his love) is painfully characteristic–and a sequence in which a medieval ball has the duo dancing to rock music seems nothing more than a variant of the inevitable prom episode so prevalent in today’s teen flicks. (It doesn’t help that William is clothed in a tunic hastily sewn together from tent rigging–a joke that seems to point back to the scene in “Gone With The Wind” when Scarlett made a dress from her curtains, which was far better parodied in an old Carol Burnett show than it is here.)

In spite of its rather puerile, even condescending approach, however, “A Knight’s Tale” will probably be a big hit with summer audiences looking for familiar fare in an unusual setting. Certainly the cast is attractive, and does as well as the material allows. Ledger, with his hair restored to the braids he wore in the 1997 Fox series “Roar,” is an affable, charismatic lead, adept at both slapstick and scenes of derring-do (though even he seems uncomfortable in the banal romantic moments). He’s ably abetted by Mark Addy and Alan Tudyk as the peasant buddies who aid him in succeeding at his impersonation of a nobleman; their coarse, frat-boy brand of humor is bound to appeal to the target viewers. Rufus Sewell scowls and raises his eyebrows on cue as Adhemar, while Shannyn Sossamon looks fine as Jocelyn, although as written the character seems too petulant and shrewish to be completely likable. Paul Bettany is the personification of broadness as Chaucer; one must sympathize with his understandable embarrassment during the all-too-frequent moments when he has to appear in the nude (only from the rear, of course). Laura Fraser is very nice as a female smithy who makes a lighter suit of armor for William (indeed, one might think that she’d be a better companion for him than Jocelyn), and Christopher Cazenove wrings what sniffles he can from the role of Thatcher’s elderly, blind father who’s finally reunited with his son. James Purefoy cuts a dashing figure as Prince Edward of England, though from the historical perspective the doomed heir to the throne is rather poorly used.

Helgeland’s direction is efficient enough, keeping things moving and pulling off some exciting joust sequences, but he shows a penchant for overstatement, particularly in the comic scenes. Carter Burwell’s score would probably be more impressive were it not constantly being interrupted by the rock tunes, but that’s one of the necessary results of the approach Helgeland decided to take. “A Knight’s Tale” is obviously the movie he wanted to make; in comparison to what it might have been it’s disappointingly juvenile, a sort of sitcomish adventure-romance, but it’s the sort of empty-headed formula fluff that will probably sell a lot of summer popcorn.