It’s taken a long time, but Hollywood has at last made a serious movie about the difficulty of balancing career and home life. It’s a familiar story: Up early for a long workday at a large, secretive assassination bureau; home just in time to install new drapes in the living room and cook dinner for the husband. And just when you think the day’s finally through, you get an overtime call and have to head back downtown to pose as a dominatrix and kill some guy before he catches a plane.

The strain of juggling such conflicting roles and responsibilities is particularly acute for John and Jane Smith, New York exurbanites who deviate from the classic stereotype only in that they both have demanding jobs as paid assassins. If that weren’t tricky enough, they work for competing “firms,” necessitating that they keep their careers secret from one another. He believes she’s a computer troubleshooter; she thinks he’s in construction. It’s “True Lies,” but with Arnold Schwarzenegger playing both parts. No wonder they wind up in marital counseling.

That’s where we find ourselves at the opening of “Mr. & Mrs. Smith.” John (Brad Pitt) and Jane (Angelina Jolie) sit before an unseen therapist, awkwardly describing how the bliss seems to have gone out of their matrimony. The two met, fell in love, and quickly married six years earlier during a hit (it’s not clear whose) in Bogota – a happy period that is dispensed with in hurried flashback in order to focus on their current doldrums. Both work long and unpredictable hours: Jane with her best friend (Kerry Washington) in a high-tech skyscraper headquarters; John with his pal (Vince Vaughn) in a basement hideout so dingy and cluttered that it looks as though Mr. Vaughn’s “Old School” buddies broke it in as a frat house. Even when they’re home together, John and Jane don’t really talk anymore, and their silences are gradually filling up with recriminations. He doesn’t like the drapes; she’s nonplussed that he asks her to pass the salt even when it’s within his reach.

But fate intervenes before divorce attorneys have to. Both John and Jane are assigned by their respective bosses to eliminate the same target, a coincidence that puts them squarely in one another’s sights. As a result, the intended hit is botched, and both Mister and Missus are given ultimatums to annul the marriage with extreme prejudice.

What follows is a variant on the kill-or-be-killed spousal dilemma of “Prizzi’s Honor,” though with the pace, volume, and array of armaments ratcheted up considerably. In the former film, Jack Nicholson and Kathleen Turner’s need for lethal closure was the culmination of the romantic and dramatic narratives; in “Mr. & Mrs. Smith,” it is the narrative – or at least two-thirds of it.

John and Jane try to kill one another at home, at her office, at her backup office (his own base of operations is apparently too filthy to stage even a murder in),and back at home again. But following this last encounter, which begins with small-arms fire and concludes with some nasty hand-to-hand, the two discover they still have the hots for one another after all and grapple again, this time amorously. In their shot-up kitchen the next morning, they wear the abashed grins of the well bedded, and agree to turn their violent attentions on their unromantic employers.

That’s about it. The film is aggressively devoid of back-story. We never learn how either John or Jane got into the whacking business or what purposes (corporate? governmental?) their respective firms serve. Instead we get an escalating series of deadly set pieces – first between John and Jane and then between the recommitted couple and legions of faceless bad guys – punctuated by occasional flurries of wry dialogue. (“I missed you,” John declares coyly after their first mortal encounter. “I missed you, too,” Jane replies.)

It’s a pretty slender conceit for a film, but a lot of good entertainment has been made from less. Moreover, in Mr. Pitt and Ms. Jolie, you’d expect the film to have more than enough star power to fuel a heady ride – and not merely because the two of them have of late provided interesting reading in the grocery checkout line.

Which is why it’s such a disappointment that neither star is close to top form in “Mr. & Mrs. Smith.” Tabloid conspiracy theorists may make of it what they want, but the fact is that Mr. Pitt and Ms. Jolie are a bit too convincing as a couple that has tired of each other. There’s very little joy in either performance, none of the spark that enabled Mr. Pitt to steal two “Ocean’s” movies from George Clooney or made Ms. Jolie’s brief cameo the most memorable part of “Sky Captain and the World of Tomorrow.” Both performers seem slightly terse, even peevish. Mr. Vaughn tries hard to enliven the proceedings with his usual fast-talking man-child routine, but like the rest of the cast he’s confined to the margins.

Much of the fault for the movie’s brusque tone lies with director Doug Liman (“Swingers,” “Go”) and screenwriter Simon Kinberg, who try awkwardly to stitch together a few not-very-compatible genres. Criticism will doubtless focus on the film’s uneasy marriage of droll humor and massive body count – and rightly so. But equally problematic is the movie’s inability to choose between biting marital satire and tongue-in-cheek romantic romp. By the rules of genre (if not life), John and Jane’s post-coital grinning over orange juice and shrapnel should have laid to rest any question about the strength of their marriage. But it doesn’t.

Throughout their subsequent homicidal hijinks, the two bicker incessantly (and only occasionally to comic effect), leading Jane to propose that they recognize their marriage “for what it is and what it isn’t” and split up. They don’t, of course, and after a few more shootings (and bludgeonings and stabbings) Jane, facing likely death with her hubby, has pulled another 180, declaring, “There’s no place I’d rather be than right here with you.” Such fickle behavior gets to the confusion at the core of “Mr. & Mrs. Smith”: It’s not sure whether it has a cynical or romantic heart.

What follows Jane’s amorous declaration may be the most abrupt ending in the history of cinema. She and John leap out into a circle of villains for perhaps 90 seconds of John Woo-esque gunplay that culminates with only the two of them left standing. With barely a breath, the film cuts back to couples therapy, where John and Jane now deliriously announce their deep love and commitment. The slaughter of a few dozen anonymous baddies has evidently solved not only their marital difficulties but their occupational ones as well. (Don’t their globe-spanning assassination outfits have, um, other guys to send after them?) Earlier in the movie, Jane had claimed that “a happy ending is just a story that isn’t over yet.” Which is exactly what its trite, hasty conclusion makes “Mr. & Mrs. Smith” feel like.