John Smith (Brad Pitt) sits before a therapist and begins to vent about his wife, Jane (Angelina Jolie).

“There’s this huge space between us that’s filling up with everything we don’t say to each other. What’s that called?”

“Marriage,” the therapist replies.

The problems the Smiths are going through in their fifth year of togetherness (actually the sixth year, Jane constantly corrects) is typical of most couples. John views his wife as a control freak; Jane considers her husband uncommunicative and aloof.

This relationship is complicated by the fact that they are both shadowy assassins for hire who work for competing firms – and neither knows the truth about the other.

“Mr. & Mrs. Smith” is a fine wedding between salient humor and energetic action. Director Doug Liman proves the perfect choice for this project, having handled comedy (“Swingers”), comedy-action (“Go”) and straight-up action (“The Bourne Identity”).

He makes great sport of blending the Smiths’ amoral job duties with their faux suburban lifestyle. Jane stores her weapons in a high-tech display case that pops out of her oven. John doesn’t try to conceal lipstick on his collar when he returns from work; he tries to hide bloodstains.

Liman’s utter lack of sentimentality gives the film its dramatic weight. These aren’t nice people; they’re remorseless killers pretending to be nice people.

In a scene at a neighbor’s dinner party, Jane accidentally ends up holding a friend’s baby on her lap. A more conventional picture would have shown John observing the scene and having it strike some kind of emotional chord. Instead, he views it the same way she does: It’s as comfortable as if she were holding a urinating porcupine.

Adding to the fish-out-of-water aura is Liman’s old “Swinger’s” pal Vince Vaughn, who shows up as one of John’s agency buddies. It’s a quintessential Vaughn role: a fast-talking, loud-mouthed, goofball hipster. Only this time he’s a pathetic bachelor living with his mom in a home packed with all types of deadly assassin gear.

But despite Liman’s obvious contributions, this isn’t really a director’s movie. It’s more a throwback to the era of pure star power.

Hard to believe Nicole Kidman, Catherine-Zeta Jones and even Will Smith were once attached to the project, because Pitt and Jolie are so right for the roles. Nobody can wear a skintight leather dominatrix outfit while wielding a pistol with more authority than Jolie. Pitt – a Springfield, Mo., native – has a Midwestern earthiness that grounds his pretty boy appearance. And he always plays characters that are slightly dorky. There’s a running gag in the flick that no matter how fancy his combat moves are, he inevitably knocks something over while executing them.

It’s no surprise the couple that launched 1,000 tabloids started dating after they made the movie. They’re too cool and beautiful not to be together, and their chemistry is undeniable onscreen. This helps the film immensely because the plot line gives plenty of reasons for the Smiths to dissolve their marriage – both legally and mortally – yet the audience needs to pull for them to remain a couple.

The premise of “Mr. & Mrs. Smith” has been touched upon in “Prizzi’s Honor,” the Jack Nicholson/Kathleen Turner vehicle about a romantic duo of mobsters assigned to perform a hit on each other, and in “True Lies,” which starred Arnold Schwarzenegger as a secret agent who tried to keep his real identity secret from his wife. Throw in a little bit of the home-wrecking “War of the Roses” as well.

What makes the movie unique is that when these killers discover their respective secrets, it doesn’t necessarily alter their relationship.

They get into the same types of arguments. Only now instead of spats involving the color of the new drapes, they concern things like who should be the one driving the car and who should take shooting duties when being chased on the freeway by machine-gun blasting assailants.