Ivan Reitman ― whose directing career (“Ghostbusters”) is so long in the tooth that he actually has a son, Jason, directing Oscar-worthy comedies ― has his best outing in decades with “No Strings Attached,” an amusing flip of the “friends with benefits” sex-leads-to-love romantic comedy formula.

It’s a movie benefiting from another sparkling, sexy and emotionally available performance by Natalie Portman, some clever turns in situations and witty banter that isn’t shy about crossing over into “Hangover” level raunchy.

Elizabeth Meriwether’s script has that “(500) Days of Summer” gimmick, telling the story of this couple in clumps over a 15-year period. Super-smart Emma met hunky-needy Adam at summer camp, way back when, and they had a momentary fling. Ten years later, they meet again and the pretty, flirty Emma (Portman) invites Adam to “this thing” she has to go to. It’s her dad’s funeral. But dopey-handsome Adam (Ashton Kutcher, NOT cast against type) doesn’t hear the “She’s cut off from her emotions” warning bells, even when she confesses, “If you’re lucky, you’re never going to see me again.”

Another chance encounter years later leads to an exchange of phone numbers.

And then, that magical night when the boy drunk-dials the girl and something begins. But don’t call it a thoroughly modern romance. Emma, now an MIT trained doctor, won’t have that. She’s busy. She’s guarded. And she’s interested in sex ― somebody “in my bed at 2 a.m.” ― and nothing more.

They have their romps, but snuggling and the like ― real intimacy ― scares her off. So for Adam, the chase is on.

Portman, almost certainly an Oscar nominee for “Black Swan,” carries this movie with her warmth and her wicked way with an incredibly crude come-on. Kutcher is better at bringing the funny that in carrying the emotional weight. Reitman didn’t suddenly evolve into a warmer, deeper filmmaker, either.

But the director surrounds his leads with funny people saying witty things. Adam’s best friend (Jake M. Johnson) mocks him for giving his lady love a gift of balloons ― “Who do you think you are, the old guy from ‘Up’?” Kevin Kline plays Adam’s has-been TV star dad, a lecher who thinks nothing of taking up with one of Adam’s ex-girlfriends. Lake Bell is the leggy but awkward and lovestruck co-worker at Adam’s job. (He’s a production assistant on a “Glee”-like high school musical show.)

And the wonderful Greta Gerwig (“Greenberg”) spices up the role of Emma’s college pal, the one who barely outgrows that sorority girl’s mating call ― “I’m so druuuuuunk.”

Whatever corners the writer Meriwither paints herself into ― and this movie seems stitched from several recent romances including “Rachel Getting Married” (Olivia Thirlby is Emma’s younger, matrimony-minded sibling) ― cute situations and cheeky dialogue bail her out. You know it’s love when the guy makes you a menstruation mix tape ― “Red Red Wine,” “I’ve got the World on a String,” and an even more obvious Leona Lewis hit.

And the sentiment ― her love of convenience, his love of love ― hasn’t grown old, through “(500) Days of Summer,” “Up in the Air” (by Reitman’s son) and “Love & Other Drugs,” though it might by the time a movie actually titled “Friends with Benefits” hits theaters this summer.

By Roger Moore

(The Orlando Sentinel )

(McClatchy-Tribune Information Services)