Ashton Kutcher should go back to “Dude, Where’s My Car?” Brittany Murphy seemed much more alive playing a sexpot opposite Eminem in “8 Mile.” In any case, the two cute kids show serious limitations in “Just Married,” a “National Lampoon’s Honeymoon Vacation” for the young at heart and juvenile of head.

Directed by Shawn Levy, whose junior-high comedy “Big Fat Liar” with Frankie Muniz and Amanda Bynes was a far more sophisticated piece, this would-be screwball comedy begins as Kutcher’s Tom and Murphy’s Sara return to Los Angeles from Venice. The increasingly rough games they play as they exit the airport — at once point he cuts down his new bride by shoving a baggage carrier at her legs, toppling her — lead to a standoff at his Dodge gas guzzler, when she prevails on him to give her a lift.

Sara, nee McNerney, comes home to a gated estate, and sobs and snivels to her snotty sister. Tom Leezak returns to a scuzzy loft, littered with empties, where he finds his conked-out best buddy, David Moscow’s layabout Kyle. When Tom arrives late at his job as a radio traffic reporter, he begins to tell his sad story to a colleague. And the slapstick flashbacks begin.

Much of the story harks back to the opening, with many a shot to the head. Sara and Tom meet when he bounces a football off her bean. On their arrival at an especially tacky honeymoon suite, he cracks her noggin against a doorjamb while hauling her across the threshold. In Venice, she hurls a marble ashtray at Tom’s skull. Clunk.

Many another accident dogs the love story and the marriage. One involves a pooch in fact, as Tom bounces a ball for Sara’s obnoxious bulldog, which flies out the window trying to fetch it (the gag calls to mind “There’s Something About Mary,” which seems a masterpiece by comparison). On the plane to Europe, Tom jams his foot in a toilet as the couple tries to join the mile-high club. At a fancy-schmancy French castle, Tom short-circuits the medieval electrical system. Then kicked out in a blizzard, he plunges their mini-car into an Alpine snowbank. An angry Mercedes-Benz driver rams the little yellow car, loaded with red luggage, over a cliff.

At last, after a horrific first night in a barbarous Venice pensione, Sara asks her father for money. The difficulties between the newlyweds arise in part from class differences. Sara is a Wellesley-educated, hugely rich WASP, known to her family as Peewee (her mother’s nickname carries a sexual innuendo, for an oft-repeated joke). The McNerneys look down on Tom’s Polish-American, community college background, and he is something of a lowbrow, preferring a Venetian sports bar to Tintorettos.

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“Just Married” becomes irritating almost immediately as it plods through its triangular story of Sara’s involvement with a persistent preppy/business hotshot played by a smarmy Christian Kane. In fact, it is almost a relief when Kutcher breathes a code line from “Dude” “sweet.” But this post-teen “Bickersons” is anything but.