Like “No Strings Attached,” the Natalie Portman-Ashton Kutcher romcom with essentially the same plot, “Friends With Benefits” pretends to upend the tired conventions of the genre while actually embracing each and every one of them. And as if that weren’t bad enough, it comes in last in a weak two-movie field.

After being dumped by their respective “significant others” (Andy Samberg and Ellen Stone) in a five-minute prologue that’s easily the funniest scene in the picture, Jamie (Mila Kunis), a NYC corporate headhunter and Dylan (Justin Timberlake), a Los Angeles website designer, meet cute when she recruits him for an editorship at GQ and drives him into the city from the airport. Before long he’s taken the job and they’ve entered into the titular relationship, both suffering from the recent breakups that have left them shy of emotional commitment but still hungry for physical satisfaction.

Where this is headed is about as subtle as a bazooka round. Though they try to keep things cool between them, they clearly begin to fall for one another. To try to make the obvious seem fresh, the script, and director Will Gluck (who co-wrote it) fill the running-time with some supposedly humorous-steamy bedroom scenes featuring the stars (both showing a discreet amount of skin) and a few “colorful” supporting characters. These include Tommy (Woody Harrelson), the ostentatiously gay CQ sports editor who’s constantly giving Dylan sage advice about relationships both gay and straight, and Lorna (Patricia Clarkson), Jamie’s free-spirited mother—a tart-talking, hippie-like refugee from the seventies, as well as Parker (Bryan Greenberg), a doctor with whom Jamie links up before yet another separation.

The last act revolves around a visit Dylan and Jamie make to his family in their beachfront California home, which introduces Anne (Jenna Elfman), his supportive sister, and her precocious son, would-be magician Sam (Nolan Gould)—as well as Dylan’s dad (Richard Jenkins), who’s suffering from Alzheimers. Besides allowing for a completely extraneous sequence in which the two reluctant lovebirds wind up sitting on the famous Hollywood sign (which brings on Dylan’s acrophobia, though elsewhere we see him accompanying Jamie onto the roof of a New York skyscraper with no apparent ill effect), this sequence also clumsily inserts the inevitable obstacle to their finally getting together, a narrative necessity in this kind of cookie-cutter chick flick before the obligatory big last-moment reconciliation.

Despite the pretense of eschewing the romantic comedy cliches the lead characters make a point of disparaging (embodied in some fake “movie-within-a-movie” footage (featuring Jason Segel and Rashida Jones), this is all remarkably standard-issue fare, and a viewer would have to have lived in a cave not to foresee every plot turn far in advance. “No Strings Attached” was no better in that respect, but it benefited from engaging stars, some crisp dialogue and a raft of amusing supporting characters. By comparison “Friends” is drab and lifeless. Kunis has been an engaging presence elsewhere, but she comes across as brittle and harsh here—her laugh (which she’s forced to emit all too often) sounds forced and phony; Portman handled the part of the hard-driving woman far better. As for the male side of the equation, Kutcher reined his exuberance in and actually seemed charming as well as goofy; by contrast Timberlake is utterly bland, and his line readings are flat.

Nor are the supporting characters particularly well-written. Both Harrelson and Clarkson try hard to put over lines that are obviously meant to sound edgily clever but instead come across like leftovers from stale stand-up routines. As for Jenkins, he’s a fine actor, but this is a part he’s played before (in “Dear John”), and here he’s also compelled to deliver that dreary chestnut—the speech in which the addled fellow comes back to clarity long enough to tell his son that “life is short” and he should do whatever it takes to win the heart of the woman who’s clearly meant for him.

On the visual side, cinematographer Michael Grady takes advantage of both New York and California locations, and even manages to make some extra-cutesy crowd-dance scenes look better than they ought to. On the other hand, music supervisor Wende Crowley has contributed one of the most irritatingly bouncy (and overloud) pop scores in recent memory.

The relationship depicted in this movie may bring some benefits to the friends of the title, but the picture itself offers few to the rest of us.