It may be an adaptation of one of Shakespeare's most famous dramas, but "Scotland, PA" is anything but deadly serious. It's deliriously funny, fast and loose, accessible to the uninitiated, and full of surprises.

Who'd have thought murder and madness of "Macbeth" could become a black comedy set in a 1970s fast food diner? Who'd have imagined the three witches, who open the play with their ominous predictions, as pothead hippies with a Magic 8-Ball? Or Macduff, the general who swears revenge for his king's murder, as a police lieutenant played with deadpan delight by Christopher Walken? Who could have imagined TV sweetheart Maura Tierney ("Newsradio," "E.R.") would make such a deliciously conniving, yet sympathetically human, bitch-on-wheels Lady Macbeth?

Oh, pardon me. That would be Pat McBeth, wife of the most under-appreciated burger flipper at Duncan's Diner. Joe McBeth (James LeGros) -- "Mac" to his friends -- is a wage slave schmoe with what he thinks is a great lifetime ambition: To pitch his "revolutionary" vision of opening a drive-through window to his boss, who will be so impressed that he'll make Mac his new manager.

While diner owner Norm Duncan (James Rebhorn) is duly bowled over by Mac's big plan, the promotion he offers is only to assistant manager under Duncan's do-nothing teenage son Malcolm, who'd frankly rather have nothing to do with the family business.

Mac is bummed out, man. But this is the last straw for his wife, a counter girl at the restaurant. She railroads her goosey husband into staging a midnight robbery so they can murder Duncan and take over the joint. "We're not bad people, Mac," she coos. "We're just under-achievers who have to make up for lost time."

Adapted by actor and first-time director Billy Morrissette (who is Tierney's real-life husband), the devilishly clever but deceptively lowbrow "Scotland, PA" has an ear for wicked humor and a genuine complexity of character. Yet it's still a refreshingly simplified take on Shakespeare that requires no knowledge of the Bard's play to understand and enjoy -- except for one ongoing joke.

Pat's hand is burned by a splash of grease when, during the "break-in," Duncan dies by falling into the French-fry fryer. She compulsively applies ointments and wears gloves to cover the "evidence" of her involvement, but Morrissette doesn't make it clear until late in the film that the burn has healed and she's simply vexed by mad guilt that's driving her to believe the burn is getting worse.

Those familiar with the picture's source material (the opening credits include an industry in-joke title card "Story by William Shakespeare") will recognize this as a crafty twist on the Lady Macbeth's "Out, damn spot!" blood stain. But the lack of an adequate explanation before the last reel might leave "Macbeth" virgins wondering why her hand is still bothering her when months have gone by.

"Scotland" has a handful of other slightly conspicuous imperfections. A handful of scenes feel contrived (a press conference for the opening of a drive-through?) and the hippie scenes are under-rehearsed (or badly ad-libbed) -- although the hippies themselves are well cast. They're played by "Road Trip"-"Rat Race" hottie Amy Smart (also a hippie on NBC's "The '60s" miniseries), whacko playwright/performance artist Timothy "Speed" Levitch, and "Newsradio" goofball Andy Dick.

But the movie's joyously seat-of-their-pants feel inspires one to give Morrissette and his cast a little latitude. Tierney and LeGros build sublime lust, then tension, then mistrust between the McBeths as Pat turns delusional and more and more manipulative while Mac becomes paranoid and unstrung. Walken, of course, starts stealing scenes the moment he walks into the diner to investigate Duncan's murder. "I'm Lt. McDuff...Ernie," is all he says. But the way he flashes that patented, friendly but just this side of psycho smile is enough to make you grin ear to ear.

Production designer Jennifer Stewart and costume designer David Robinson ("Zoolander") deserve equal kudos for complimenting the movie's Bad Company/Three Dog Night soundtrack with a perfectly groovy, low-budget '70s vibe. Muscle cars, shag carpet, feathered rocker-dude hair, super-snug denim, Pucci print scarves and even those long, brown Virginia Slims cigarettes give "Scotland" acid-flashback ambiance.

Ultimately, though, all great Shakespeare adaptations come down to one thing: dialogue. Morrissette's script is thick with sharp but modest, well timed, fun to speak and fun to listen to, very tongue-in-cheek dialogue. But as with Shakespeare proper, it's all in the delivery.

Early in the film, another diner employee who is having drinks with Mac says he's seen Malcolm stealing from the cash register. "Hold on a minute," Mac replies. He walks over to the jukebox and gives it a swift Fonzie-like kick so the music changes to something more incidentally dramatic -- he knows something big is about to happen. He comes back to the table, fixes the guy with a perniciously curious smile and whispers, "Tell me everything..."