"No Strings Attached" isn't the first film in recent memory about people struggling to keep their emotions at bay while embarking upon a strictly sexual relationship (2010's inferior "Love and Other Drugs" beat it to theaters by two months), and it's certainly not going to be the last ("Friends with Benefits," starring Mila Kunis and Justin Timberlake, is still several months away). It may, however, go down as the most sincere of the lot. Directed with reclaimed exuberance by Ivan Reitman (2006's "My Super Ex-Girlfriend"), who hasn't made something this good in nearly twenty years, the film, like 2010's underappreciated "Going the Distance," is an all-too-rare R-rated romantic comedy that uses its rating not as an excuse to be needlessly raunchy, but instead to simply be honest and forthright about its characters and subject matter. Randy without being tawdry, sophisticated without feeling stifling, "No Strings Attached" scurries through genre conventions even as the screenplay by Elizabeth Meriwether refuses to compromise its intelligence and appreciable eye for detail. It helps all the more that the on-screen lovers in question—and the actors portraying them—have a heck of a lot of chemistry, not only with each other but with an ensemble of sparklingly written and well-cast supporting players.

They first shared a memorable encounter as young teens at Camp Weehawken, then again ten years later where a chance meeting at a college frat party led to him accompanying her to her father's funeral. Now, five years past that, Emma Kurtzman (Natalie Portman) is a hard-working medical intern who has no interest in romance, and Adam Franklin (Ashton Kutcher) works as a lowly production assistant on a "Glee"-style television series. Depressed over the discovery that ex-girlfriend Vanessa (Ophelia Lovibond) has taken up with his father, aging former sitcom star Alvin (Kevin Kline), Adam heads out with buddies Eli (Jake Johnson) and Wallace (Chris 'Ludacris' Bridges) to drink away his pain and wakes up the next morning in Emma's apartment without any memory of how he got there. They didn't sleep together, it turns out, but they have by the time he leaves to go home. When Emma suggests they keep having sex—anytime, anywhere, only a text or phone call away—Adam is elated to oblige. Eventually, however, their burgeoning hearts are bound to get in the way of their more carnal urges. Adam embraces the idea, but Emma, who has never been a touchy-feely kind of person or seen herself as the kind to settle down with someone, isn't so sure she's ready to take the next step.

Caustic before gradually letting down its defenses, "No Strings Attached" is as exceedingly modern in its premise as it is traditional in its destination. True love, no matter how it might begin, is funny like that. In bringing together Emma and Adam, first as acquaintances, then as bedmates, then as potentially something more, the picture confidently does so without much tug or pull on the script's natural feel, low-key tone, and frequently very amusing sensibilities. Emma and Adam are smart, ambitious individuals—she works eighty hours a week at the hospital and is highly career-oriented, while Adam preps a teleplay he has written for the show he works on, hoping it will finally get him noticed—but they're also engaging, unpretentious, and just the kind of characters one is happy to watch for the better part of two hours. They're not above enacting mistakes—Emma long ago put up a tough exterior for her family and now, as an adult, is having trouble letting this go for the chance to be genuinely happy—but the decisions they make and actions they take feel believable rather than as a strained excuse to merely bring conflict to the story. We don't always relate to Emma's point of view, but we understand it, just as we understand why Adam is so hurt when he tries to express himself and is shot down.

Natalie Portman has lately entered a renaissance period in her career, receiving well-deserved award notices for 2010's "Black Swan" and, only three weeks into the new year, having already turned in two more strong performances with thoughtful indie drama "The Other Woman" and now this. "No Strings Attached" is certainly more lighthearted than the aforementioned pictures, and while Portman adjusts nicely to her more humor-based surroundings—she unexpectedly garners quite a few laughs—she also takes the part of Emma just as seriously. Ashton Kutcher (2010's "Killers") is on more familiar terrain as Adam. He has played this kind of role—that of a good-hearted puppy dog of a romantic lead—more often than not, but if Kutcher is left generally unchallenged, that doesn't take away how good he is at it. Indeed, Kutcher meets the formidable Portman step for step, and the two of them have an infectious camaraderie that really leaves one caring about them. The first official date they go on, complete with an unpredictable game of miniature golf, dinner at a charming diner, and sight-seeing around Los Angeles, is lovely in the way it pays attention to them and their behavior. Even when Emma is saying that she doesn't want things to go further, you know that she really does. Also a bright spot in the furthering of their relationship comes when Emma agrees to go with Adam to meet his father and ex-girlfriend for a big announcement; where this scene goes, and how Emma has no choice but to give the inane couple a piece of her mind while simultaneously sticking up for Adam, is both immensely sweet and hugely funny.

Side parts in this type of movie are usually throwaways, mostly consisting of friends and co-workers whose sole job it is to be confided in by the main characters. Here, those typically thankless roles are enlivened by sharp writing and performers who realize there are no small parts, only small actors. One after the other, they blow in and, without seeming to even try, threaten to steal the show. The irresistible Greta Gerwig (2010's "Greenberg") and perfectly acerbic Mindy Kaling (2007's "License to Wed") are a treat as Emma's pals and roommates Patrice and Shira, the former shocked and charmed when her stream of bad relationships end once she meets Adam's respectful buddy Eli, and the latter realizing how badly she needs a man after seeing all the couplings going on around her. As the high-strung Lucy, an executive producer with eyes for Adam but rocky social skills, Lake Bell (2009's "It's Complicated") turns in an inspired comic turn that nonetheless never becomes a caricature. Ophelia Lovibond comes closer to just that as flighty ex-girlfriend Vanessa, but one suspects there are plenty of real people just like her, self-centered and blissfully unaware of her own idiocy. Lovibond gets a laugh with nearly every line she delivers.

Also in fine form, making the most of their screen time: Olivia Thirlby (2008's "The Wackness") as Emma's younger in-love sister Katie; Talia Balsam (2010's "Conviction") as Emma's caring mother Sandra, and Abby Elliott (TV's "Saturday Night Live"), doing a killer Drew Barrymore impression, as Joy, a kooky young woman Adam meets in a bar who quickly reveals she's more interested in someone else altogether. Less impressive, though no fault of the actors', are Cary Elwes (2010's "Saw 3D"), whose Dr. Metzner is awkwardly thrown into a few scenes with nothing really coming of him, and Ben Lawson, as Emma's fellow intern Sam, pitted as an adversary for Adam with, again, not much coming from it. When Sam taunts Adam in one scene by saying that he's the kind of guy Emma will marry—someone who can take care of her—it doesn't make sense; Adam, after all, shares a bachelor pad in the Hollywood Hills with Eli, and isn't exactly doing badly for himself. Finally, Kevin Kline (2008's "Definitely, Maybe"), as Adam's sitcom star father Alvin, begins the film as a ridicule-worthy oaf before a helping of wistful pathos transforms him into a multidimensional presence. Two late scenes—one involving a birthday serenade, and another a much-needed heart-to-heart where Alvin makes clear how proud of Adam he is—are especially touching.

Taking advantage of an eclectic soundtrack (including, outrageously, an impeccably chosen "period mix" that Adam makes for Emma's time of the month) and all the lovely scenery Southern California has to offer, "No Strings Attached" has its share of solid tech credits and handsome production values. As for the film proper, it tells a story that is, at once, contemporary and old-fashioned. Director Ivan Reitman, perhaps reinvigorated by the work of his filmmaker son Jason (2009's "Up in the Air"), avoids pandering to audiences the way most romantic comedies do—only the end credits montage could be construed as overstepping this fine line—and instead centers on the humanity within his characters. At the center of things is Adam and Emma, a top-notch screen couple amidst a script that knows what it's doing. Near the end, Adam utters a line in the vein of, "Nobody puts Baby in a corner," from 1987's "Dirty Dancing" and, "You complete me," from 1996's "Jerry Maguire"—a romantic hallmark that most films try and fail to pull off. Without just the right touch and delivery, dialogue like this can come off as shamelessly cornball and ripe for unintentional laughter. Not here. "No Strings Attached" earns this climactic amorous flourish. It's a great moment in the first wholly worthwhile studio release of 2011.