Six years ago, Shrek stood the fairy-tale world on its ear with its Monty Pythonesque take on the oevure of the Grimm Brothers, Charles Perrault and Hans Christian Andersen. An uncouth ogre (Mike Myers’ Shrek) was cast in the role of knight-errant rescuing a fair princess (Cameron Diaz’s Fiona) — and, while the Beauty and the Beast conflict at least partly led to the inner humanizing of the beast, the ultimate outcome was the outward ogrification of the beauty. Hey, it’s who you are inside that counts.

Shrek 2 continued the farcical subversion of fairy-tale archetypes, with a cynical, preening Prince Charming and a Lady Macbeth-like Fairy Godmother conspiring with Fiona’s royal father (John Cleese) to break up Shrek’s marriage and arrange a more conventional happily ever after for Fiona.

Deep down, in spite of their cheerful vulgarity and skewed sensibilities, both Shrek and its sequel had real heart. There was genuine sweetness in Shrek and Fiona’s relationship, and even Fiona’s flawed father was given a chance at redemption and heroic sacrifice for the sake of his daughter’s happiness.

Shrek the Third continues the deliberate bad taste that is the franchise’s hallmark, with the usual hit-and-miss results. Doubtless kids will scream when a curtain pulls back to reveal an elaborately costumed Shrek, unable to bend his arms in an Elizabethan monkey suit, getting his butt vigorously scratched by a page with a cane.

Grown-ups are more likely to get a kick out of an outrageously extended death scene that sets the plot in motion, leaving Shrek as heir apparent to the throne unless he can convince the next heir in line to take the job.

What’s missing is the heart that leavened the first two films. A perfunctory effort to take Shrek and Fiona’s relationship to the next level — into parenthood — is neglected for too much of the running time. And efforts at poignancy between Shrek and the next heir in line, a callow young Artie Pendragon (Justin Timberlake), don’t quite click.

Yes, that Artie Pendragon. Shrek the Third ventures into King Arthur territory, with the once-and-future king-to-be as a disaffected, socially inept high schooler, a New Agey, pop-psych Mr. Merlin (Python veteran Eric Idle), and walk-ons with bullying jock Lancelot and popular, spoiled Guinevere.

Plotwise, Shrek’s quest to bring Artie to Far Far Away to meet his royal destiny whether he likes it or not recalls his mission in the original to bring Fiona to Lord Farquaad. Alas, the sensitive but petulant Artie  never threatens to become as engaging a character as the feisty Fiona, and efforts to develop a kind of big-brother bond between Shrek and Artie lack the modest appeal of Shrek and Fiona’s romantic relationship.

Since both of the first two Shrek films featured a redemptive twist, I took note when the villainous Prince Charming (Rupert Everett), having fallen on hard times, shows up at the Poison Apple (sort of a villain’s pub) saying that he’s looking for “redemption.”

But the sort of redemption Charming is looking for turns out to be the strictly non-moral sort. What he really wants is to topple Shrek, claim the throne, and get the girl — by any means possible.

In the name of this happily ever after, Prince Charming rallies the villains of all fairy tales — a motley crew that includes Captain Hook, Rumplestiltskin, Cyclops (who is technically mythological, not fairy tale), and the Headless Horseman (who is technically legendary, even in his own story) — to rise up and seize Far Far Away for their own.

“There are two sides to every story,” Charming argues, “and one side has not been told! Who’s ready for their happily ever after?”

Well, not exactly. Charming’s exact plot has actually been tried before, and the story was told in another CGI fractured fairy tale earlier this year: the forgettable Happily N’Ever After. One might hope that the Shrek franchise, a pioneer of the genre, might do better than this also-ran — and it does, I guess, but not by much.

Beyond that, the satiric riffing on fairy-tale conventions is now well past the point of diminishing returns. There’s still humor to be mined in, say, Pinocchio’s efforts to avoid being caught in a lie. But where in Shrek 2 having Larry King voice an overtly masculine Ugly Stepsister was merely a risqué sight gag, here she’s a significant supporting character, notably upping the yuck factor.

The filmmakers still land a few telling jabs on the fairy-tale canon. I cracked up when Snow White, Cinderella, Rapunzel and Sleeping Beauty matter-of-factly prepare to escape from imprisonment by … hunkering down and waiting to be rescued.

As a father with two daughters, I have some sympathy for the feminist critique of passiveness in fairy-tale heroines.

But then every one of fairyland’s “super-hot princesses” (Snow White’s term) turn out to be spoiled, catty suburban-princess types — or worse.

And Prince Charming is still a cad.

And every villain (except Charming) is merely misunderstood, and willing to give it all up at the first overture of understanding.

And King Arthur himself is a dweeb whose great coming into his own involves nothing more heroic than a cornball speech.

At some point, the Happily N’Ever After syndrome kicks in: If our heroes and villains are as banal as this, why should we care?

However skewed the humor is, there needs to be that remaining element of sincerity and redemptive sacrifice. Without it, subversiveness has nothing to push against.

I won’t go so far as to say the first two Shrek films had a lot of heart. But even a little heart, I now realize, goes a long way. Shrek the Third hasn’t got it.

Steven D. Greydanus is editor and

chief critic of DecentFilms.com.