What do you call a shameless imitation of a shameless imitation? A rip-off squared? An homage once removed? Whatever label you choose, it can be comfortably affixed to “A Lot Like Love,” an Ashton Kutcher-Amanda Peet vehicle that is a lot like “When Harry Met Sally,” which was itself a lot like “Annie Hall.”

Sadly, the resemblance does not extend to quality. Indeed, those with a scientific turn of mind may take the devolution from “Annie” to “Harry” to “A Lot Like Love” as yet another demonstration of the Second Law of Thermodynamics, which says that in a closed system (an apt description of Hollywood if ever there was one) there is a tendency toward entropy – in this case, from acknowledged classic to memorable cable-television staple to dim, flabby dud.

“A Lot Like Love’s” primary mistake is that, rather than borrow the elements that made “When Harry Met Sally” successful – the mismatched pairing, the evocative Big Apple setting, and most of all the top-flight dialogue – it merely apes its forebear’s plot: Two 20-somethings meet on and off over the course of several years, become friends, and eventually fall in love. It’s a perfectly fine premise for a movie, but its execution here speaks volumes about the sorry state of romantic comedy.

Like Harry and Sally, Oliver (Mr. Kutcher) and Emily (Ms. Peet), first meet on a trip to New York. But where the former couple had plenty of time to get to know (and loathe) one another on a chat-heavy drive from Chicago, the latter don’t exchange a single word on their flight from Los Angeles, instead settling for a few interested glances followed by a bout of anonymous sex in the airplane restroom. Yes, this is contemporary Hollywood’s idea of how to find your lifelong soul mate.

Following their touching induction into the mile-high-club, Oliver and Emily hook up another half-dozen or so times over the next seven years, in New York, Los Angeles, and San Francisco. The usual reason for the rendezvous is that one or the other of them has suffered an unpleasant breakup; the usual result is a few minutes of utterly forgettable dialogue followed by their tumbling back into the sack together. (In one case, Emily passes out on a toilet before they can do so.) A dead mother (hers) and a deaf brother (his) are thrown into the mix to offer some semblance of the depth and affection that are otherwise in scant evidence.

In remarkable contrast to the literate movies whose DNA it has stolen, “A Lot Like Love” is almost pre-verbal in its appeal. Among its funniest moments (and, rest assured, we’re grading on a curve here) are Oliver and Emily giving one another the silent treatment, sticking straws up their noses, and singing along in the car to Chicago’s “If You Leave Me Now.” Such antics may seem pure genius when performed by your 11-year-old cousin (particularly if you, too, are 11 years old), but are they really the best a $30 million film can manage these days?

If one thing saves “A Lot Like Love” from disaster – and I’m not sure it does – it’s an easy chemistry between the leads, though one that owes little to Mr. Kutcher’s performance. While he was a suitable enough foil for Bernie Mac’s comic intensity in last month’s “Guess Who,” Mr. Kutcher fails to supply the energy required of him in this more balanced pairing, especially in the early scenes. His Oliver is simultaneously too dull to merit Emily’s long-term longing and too passive to be convincing as a self-made striver who founds his own internet company. (Kal Penn, of “Harold and Kumar Go to White Castle,” meanwhile, is utterly wasted as his no. 2.)

Fortunately, Ms. Peet carries off the role of Emily with enough spark that even Mr. Kutcher gradually lights up. With her wide mouth and sly eyes, Ms. Peet displays a mischievous enthusiasm that enlivens even otherwise lame gags – as when she ostentatiously feigns choking in the middle of a restaurant (another bit cribbed none-too-subtly from “When Harry Met Sally”). Since running away with “The Whole Nine Yards” five years ago, Ms. Peet has had altogether too little opportunity to showcase her sexy comic appeal. “A Lot Like Love” might have provided that opportunity, had it only featured something a little like a script.