Off-screen chemistry does not necessarily translate onto the screen. Kidman and Cruise? Ben and J. Lo? Enough said. As for Brad Pitt and Angelina Jolie in "Mr. and Mrs. Smith," that's an entirely different story. From the moment their characters first meet--a chance encounter in appropriately sultry Bogota--the erotic sparks light up the screen. Sleek and graceful, two of a rarefied kind, they size each other up, come to the quick, obvious conclusion that they were made for each other, and head right to bed. "Mr. and Mrs. Smith" actually begins years after this lusty first round, when John and Jane Smith have settled into a dull, dissatisfied suburban marriage. Here they are, facing a marriage counselor, bickering about the drapes and wondering why their marriage has gone flat. If you have a hard time swallowing these impossibly glamorous movie stars as a bored, ordinary suburban couple, that's because they are nothing of the kind. In fact, both are highly paid assassins, and so well versed in the art of deception that they have kept their real identities a secret even from each other. They learn the truth in a most disconcerting fashion, when each is given orders to make a hit on a rival assassin. In other words, their orders are to kill each other.

Given the acrimony that has settled into the Smiths' marriage, both take to their new jobs with a certain relish. But does John know that Jane is on to him, and visa versa? The ambiguity doesn't survive their first at-home dinner, where the carving knives get an extra workout. Suddenly even the most banal terms of endearment ("Honey, I missed you") take on hilarious double meanings.

Imagine "The War of the Roses" remade as a James Bond fantasy, with appropriately high-tech weaponry, and you have some idea of what Doug Liman's heavily armed comedy has in store. It's a high-wire act, pitched above a gaping chasm of implausibility, and the remarkable thing is how well Liman ("The Bourne Identity") and his red-hot stars sustain the joke. It's preposterous, but Liman gives it such a seductive, playfully hip texture that you happily embrace the fantasy. Jolie and Pitt's spontaneous, playfully lusty rapport has an improvisational feel that gives their most outlandish stunts a veneer of verisimilitude.

It's the conceit of Simon Kinberg's screenplay that a little life-and-death therapy can put the fizz back into the Smiths' marriage--assuming they can survive. Since they can't spend the entire story trying to bump each other off, at a certain point they realize they'd rather f--- than fight, so they team up and find that there are plenty of other folks who'd like to see them both dead. "Mr. and Mrs. Smith" works far better as a knockdown, drag-out romantic comedy than as an action movie. The obligatory table-turning plot twists in the third act are neither convincing nor particularly interesting. And there were more than a few moments when I hadn't a clue what was going on: what was the warring couple doing at a construction site where Mrs. Smith threatens to blow up an elevator carrying Mr. Smith? (Pitt's escape steals a trick from another of his movies, "Ocean's Eleven"). Romantic comedies don't usually have such a massive body count, but Liman doesn't make the mistake of taking any of it too seriously.

With the notable exception of Vince Vaughn--frazzled and funny in a supporting role as Pitt's mother-dominated boss--Brad and Angelina are the whole show here. They bring out the best in each other. On her own as a superheroine in the Lara Croft movies, Jolie couldn't quite transcend the B-movieness of the enterprise. Pitt has often worked best when he's part of an ensemble, not alone at stage center. As Jane and John Smith, they've hit their movie-star stride. They complete each other.