Matthew McConaughey and Sarah Jessica Parker might not seem an immediate on-screen love match, but the producers of “Failure To Launch” don’t seem overly concerned. When Mr. McConaughey’s character brings his new girlfriend (Ms. Parker) to get his friends’ approval, his buddy Demo (Bradley Cooper) sums up the priorities of the film quite well: “Good spatial awareness and great hair. She’s a keeper.”

Mr. McConaughey and Ms. Parker have been equipped here with matching tans,sparkling veneers,and hair extensions. As they sparkle and shine their way through the film, it hardly seems important whether they work as a pair. This romantic comedy, like many star vehicles, is less concerned with building a believable love story than showcasing the talent demanding the largest salaries.

It almost works. Mr. McConaughey is tailor-made to play Tripp, a 35-year-old charmer who refuses to let something as insignificant as his increasing age pry him from the warmth of his parents’ home. Ms. Parker, still struggling to wrench herself from the persona of Carrie Bradshaw, is Paula, a woman who disturbingly has made a career of using herself as bait to get “adult children” to leave the nest.

While Mr. McConaughey shines in any role that allows him to strut shirtless and show some charm, Ms. Parker gets the short end of the stick. Though her trademark tresses and patented strut get her pretty far into the role, trying to portray an intelligent woman who fails to see the moral dilemma in having parents pay her to date their sons is a difficult task.

But from the premise of a successful 35-year-old male living at home with his parents to the idea of attractive men attending yoga class, the film is awash in implausible scenarios.

As if prepared for audiences to shy away from the lead romance, the filmmakers of “Failure To Launch” have packed the script with great one-liners, non sequiturs, and strange animal antics. Some work better than others. A placid dolphin taking a bite out of Tripp during a surfing trip almost works, but a similarly inclined lizard that laughs at Mr. McConaughey’s travails is regrettable.

The unabashed interest in pushing all the comedic buttons allows for some lenience that would trip up a more earnest film, such as the purposely shameless inclusion of young Tyrell Jackson Williams as a multicultural, heart-tugging addition to the cast. But there are limits to this approach. Kathy Bates and Terry Bradshaw are erratic as Tripp’s parents. And the unfortunate image of Mr. Bradshaw naked will stay with me always.

At least there is plenty of eye candy on display. As Tripp’s best friends, Bradley Cooper and Justin Bartha are charming and provide extra heartthrob material for easily distracted moviegoers. But it is Zooey Deschanel who steals the spotlight. Ms. Deschanel is making a career of breathing life into boring, brooding characters. As a girl obsessed with killing a mockingbird who’s never heard of Harper Lee, she has a lot working against her. That she makes Paula’s roommate Kit the most endearing character in the film is a testament to her abilities.

The creaking machinery of today’s romantic comedy tropes appears a bit too often for the audience to get swept up in the budding relationship of Tripp and Paula, but with a strong cast willing to go after every disparate plot-point, “Failure To Launch” gets through to the final credits.

Is it necessary to craft a convincing love story for a romantic comedy to succeed? Maybe not. But as Mr. McConaughey’s most memorable character once quipped in “Dazed and Confused,” it’d be a lot cooler if it did.

mkeane@nysun.com