Alfred Hitchcock almost saves “Diary of a Wimpy Kid — The Long Haul.”

Keeping in mind, of course, that the movie’s for pre-tweens — children who might not have been exposed to any of the scads of “family vacation disaster” comedies that preceded it — and is thus graded on a curve bending towards the unsophisticated.

But for parents, and parents who have imposed Hitchcock on their pre-teen children, there’s a saving grace amidst all the poop/mudhole/piglet/parental sing-along-to-the-Spice Girls gags.

“The Long Haul” has the best “Psycho” shower scene send-up ever filmed. Well, after “High Anxiety.” And this one, at least, is utterly unexpected.

This is a “Wimpy” reboot, with a new look-alike “kid” (Jason Drucker of TV’s “Every Which Way”), a new, dopier-funnier drummer/brother Rodrick (Charlie Wright of “We’re the Millers”) and a couple of big-name upgrades as parents — Alicia Silverstone (“Clueless”) and Tom Everett Scott (“That Thing You Do”).

It starts with our Wimpy narrator becoming the victim of a viral video when he tumbles into a kiddie foam-ball playpit and freaks out as he comes up with “diaper hand,” and then it puts the lad on the road for four days with his family as they drive to granny’s 90th birthday celebration in Indiana. Or to a nearby video game convention, if Greg can figure out how to pull that off.

There are more of moronic Rodrick’s efforts to torment/mentor his kid brother and disappoint his parents. Mom (Silverstone) sends the lads into a store with a shopping list.

“I couldn’t read your curly/oldie fashioned writing!”

“You mean CURSIVE?”

And when they stop for a country fair, Greg gets stuck with his toddler littlest brother, and eventually, a piglet.

The shower scene in question kicks off a livelier third act, as Greg — covered in mud — is trapped in the hotel bathroom shower of their nemesis family as their “Beardo” patriarch (Chris Coppola) uses the facilities. The wimp’s discovery — after a gross and kid-friendly bowel movement — is a shot-by-shot recreation of Hitchcock’s most famous screen sequence, from ripped shower curtain hooks to mud circling the drain. Kudos to Brit director David Bowers for the effort.

The scattered laughs that precede it (Rodrick lines, mostly, Silverstone and Scott singing along to “Wannabe”) don’t add up to much, and the movie lacks the wince-with-recognition middle school spark that marked the first film in this tweenage franchise.

But even if you drop the kids off, come back in time to duck in for a quick shower. It makes “The Long Haul” totally worth it — almost.

MPAA Rating:PG for some rude humor

Cast:  Jason Drucker, Alicia Silverstone, Tom Everett Scott, Charlie Wright

Credits:Directed by David Bowers, script by Jeff Kinney and David Bowers, based on Kinney’s book. A 20th Century Fox release.

Running time: 1:30