There's a troubled marriage at the heart of Mr. and Mrs. Smith, but it isn't that of the title characters, John and Jane Smith (Brad Pitt and Angelina Jolie), a pair of high-powered assassins who keep their true selves a secret from one another. The uneasy alliance here is between the two genres that director Doug Liman and screenwriter Simon Kinberg have joined together: a dark, dry comedy about modern marriage and its discontents, and an enormous action film filled with rocket launchers and incendiary devices. Sometimes you wonder how two such opposites ever ended up together. But when they connect, and all those bombs and bullets begin to mirror the frustrations and frictions of wedded bliss, Mr. and Mrs. Smith is dynamite.

When first we meet the Smiths, it's at the office of a marriage counselor-not, they are quick to assure him, that there's anything wrong. After all, they live on a lovely tree-lined street in an upscale New York suburb, their perfect home stuffed to the gills with tasteful furnishings and stainless steel appliances.

Outward appearances, however, are just about all the Smiths have going for them. John can't remember how long they've been married; Jane can't remember the last time they had a decent conversation, much less decent sex. A flashback to their first meeting-a world away from the 'burbs in Bogota, Colombia, where they're both "on assignment," something they each fail to mention-lets us know just how much things have cooled off between this once-torrid pair.

When romance dies, who pulls the trigger? One of the chief suspects in Mr. and Mrs. Smith is marriage itself, or at least the kind of marriage in which passion is slowly done in by domesticity, the daily grind and excessive home decorating. After the Smiths are assigned to kill one another (a task they take to with relish), Liman has great fun shooting holes through just about every inch of their dream house, destroying the illusion of a happy home in order to save the people trapped inside it.

But the real culprit, Liman suggests, isn't so much the familiarity of marriage (which can breed contentment as well as contempt) as the fallacy that getting married means giving up your identity, submerging your strengths as well as your flaws. Only when the Smiths stop lying and start fighting-first each other, and then the various forces that lay siege to love-does their marriage come roaring back to life, guns blazing.

Jolie and Pitt make an appealing Bonnie and Clyde, and not just because of their killer looks. Whether they're exchanging insults or gunfire, they understand that the true nature of their assignment is farce, as does Vince Vaughan, who's on hand as John's motor-mouth partner. It's an especial pleasure to watch Jolie, who's usually larger than life in all ways, underplay her scenes without sacrificing any of their bite.

Had Liman managed to do likewise, had he found a way to wed the playful spirit he brought to Swingers and Go to the cool suspense of The Bourne Identity, Mr. and Mrs. Smith might have been a better picture all around-not only a sharper farce, but also a tighter action film. But here he plays it big and broad, threatening to make the story one overlong gag in which all the accoutrements of upper-middle-class married life are blown to smithereens. When the romance of this film dies, the finger on the trigger is Liman's.