"No Strings Attached" is a high-concept romantic comedy that gets off to a very funny, quite sexy start, but then lapses into auto-pilot for its remaining 90 minutes.

It ultimately adds up to a more conventional, less interesting version of  "(500) Days of Summer."

An amusing flashback device opens the movie - Adam and Emma meet at summer camp as kids, again at a college frat party, and finally in Los Angeles as adults. He (Ashton Kutcher), the son of a famous sitcom actor, works as a production assistant on a TV show, while she (Natalie Portman) is a harried medical resident.

They decide they're too busy for a relationship- and she, not he, is a commitment-phobe anyway - so they go for no-strings, sex-only relationship.

The opening device is cute and there are some funny moments early on. But after about ten minutes the movie starts meandering, and never stops until the closing credits, making sure to sell out both of its main characters along the way.

All of the beats of the relationship are telegraphed and obvious, and throughout it all, nothing particularly surprising or funny happens.

Besides, we already saw this same story - told much more realistically, and creatively, in "(500) Days of Summer."

The movie also struggles in handling a deep supporting cast of likable performers- Kevin Kline, Greta Gerwig, Jake Johnson, Mindy Kaling, Olivia Thirlby, Talia Balsam, Cary Elwes and, yes, Ludacris - who are given not a whole lot to do. 

Now, raunchy romantic comedy is a genre that's quite difficult to pull off.

The best ones- "There's Something About Mary," "40-Year-Old Virgin," "American Pie" - combine legitimate laughs and daring with heart.

The worst ones aren't funny and just plain come across as gross.

"No Strings Attached" isn't that bad, but it still struggles with the shifts in tone. It reminds me a lot of last year's Justin Long/Drew Barrymore movie "Going the Distance," another conventional high-concept romcom that packed in ultra-raunchy dialogue that always came across as forced.

What's hard to believe is that Ivan Reitman directed this - the guy who made "Ghostbusters," "Stripes," "Dave," and numerous other seminal comedies of the '80s and '90s.

Another disappointment is Kevin Kline, as Kutcher's father- between this, 2008's "Definitly/Maybe" and last year's "The Extra Man," it's really unfortunate that this great actor has been reduced to playing the same pathetic aging lothario in every one of his movies.

Of the supporting cast, the standouts are Kaling - Kelly from "The Office," who really should start getting more movie roles about now- and Greta Gerwig from "Greenberg," although neither has nearly enough screen time to establish a character.

As for the stars, after the intensity of "Black Swan," it's sort of jarring  to see Portman in something as vapid and and inconsequential as this. 

Kutcher? It's not quite so jarring.

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Roll credits:
 "No Strings Attached"

Directed by: Ivan Reitman

Starring: Ashton Kutcher, Natalie Portman, Greta Gerwig, Mindy Kaling, Kevin Kline, Olivia Thirlby, Jake Johnson.

The Silver Screen rating: 1.5 stars (out of 5)