The first thing you hear is laughter, and it freezes the blood. The loud peals of offstage merriment that begin 'Mr. & Mrs. Fitch,' the shrill new comedy by Douglas Carter Beane that opened on Monday night, are made by its title characters, an urbane husband and wife who are terribly, terribly amused by each other. The convulsed couple in this play, directed in hard-driving style by Scott Ellis at the Second Stage Theater, are portrayed by the formidable team of Jennifer Ehle and John Lithgow. Yet you don't feel like laughing with them. All that falling-down, choking, roaring ha-ha-ha stuff echoes with desperation, and I don't think it comes from the characters. It's the sound of two astute, talented and probably miscast performers trying hard to convince themselves and their audience that really, honestly, this is all so much fun. Might I just say here that as much as I admire Dashiell Hammett, I feel he has a lot to answer for. This pioneer of hardboiled detective fiction was also the creator, in the early 1930s, of Nick and Nora Charles, the chic, martini-loving husband-and-wife sleuths. Incarnated on screen by William Powell and Myrna Loy, the Charleses became the template for marriage as a sophisticated fest of cleverness, and they have been copied ad infinitum, with ever-diminishing returns. Mr. and Mrs. Fitch -- which, by the way, is how they address each other -- are descended from Nick and Nora (with distinct nods to the irritable, madcap lovers of Noel Coward's 1930 comedy 'Private Lives'). A pair of celebrity journalists (he writes a gossip column), they live to quip, to party and to crack each other up with devastating putdowns of the shallow, corrupt people who make up their gilded world. This being the 21st century, Beane -- author of the delightful 'As Bees in Honey Drown' and 'The Little Dog Laughed' -- has made some variations on the usual formula, and they at first seem vaguely promising. Fitch is bisexual and needs Viagra to perform his husbandly duties (which are necessary, as Mrs. Fitch wants a baby). And with their culturally literate cleverness and Jazz Age antics (they'll Charleston at the drop of a pun), the Fitches are presented as anachronisms in the age of the personal computers they so contemptuously use. Beane flirts with the idea of such posturing as a refuge for misfits who have turned a world that might reject them into one long, shared joke. But he doesn't develop the notion with conviction. Instead, he pursues a half-baked plot about how the Fitches, desperate to revitalize their careers, employ the mighty information-spreading powers of the Internet to create a fictional celebrity who soon takes on an autonomous life of his own. That story line is, as Fitch observes, out of 'one of the minor novels of Evelyn Waugh.' But even at his most surreal, Waugh lent a persuasive, self-contained logic to how his fantastical parodies played out. So, for that matter, did Beane, in developing the celebrity scams that fueled 'Little Dog' and 'Bees.' The central deception of 'Mr. & Mrs. Fitch,' on the other hand, feels dashed off, like something from a first draft its author meant to return to. Beane might well have heeded Fitch's admonition to his wife, after she makes a particularly implausible suggestion: 'I'm going to find a drawing board and send you to it.' Of course, lines like that one (admittedly, not very good) are the real raison d'etre of 'Mr. & Mrs. Fitch.' Along with Paul Rudnick, Beane is American theater's best living exponent of the exploding epigram, and some of the jokes here are eminently quotable. ('You know, theater, that thing that movie people do when they want to announce they're available for television.') But for every zinger, there's at least one clunker. (The first act curtain line: 'When life gave me lemons, I made citron presse.') And the unceasing attempt to stay witty takes its toll on both the audience and the overworked two-member cast, whom Ellis keeps in grinding high gear. Ehle looks smashing in Jeff Mahshie's woman-of-the-world outfits. But her voice keeps pushing uncomfortably into upper registers. Lithgow, an expert in conveying embarrassment (remember 'M. Butterfly'?), does so here inadvertently. As well he might, since it turns out that Mr. and Mrs. Fitch have stern liberal consciences and bona fide artistic ambitions that include a novel called 'America: A Prophecy.' We are evidently meant to take them as pure-of-soul knights errant in a land of dirt, who wind up fighting for Truth from their sumptuous, how-can-they-afford-it duplex loft (designed to the hilt by Allen Moyer). Because Beane has been such a canny social satirist in the past, I kept hoping he would pull the Aubusson rug out from under the Fitches' pretensions, both the naughty and virtuous kind. Such hopes were dashed when Mrs. Fitch, recalling the night she met her husband-to-be, said: 'You told me about your work and we laughed. And we laughed. And we laughed and we laughed and we laughed and we laughed.' The Fitches would be a lot easier to like if they were the sort of people who stopped after that first 'We laughed.' PRODUCTION NOTES 'MR. AND MRS. FITCH' By Douglas Carter Beane; directed by Scott Ellis; sets by Allen Moyer; costumes by Jeff Mahshie; lighting by Kenneth Posner; sound by Bart Fasbender; music by Lewis Flinn; associate director, Evan Cabnet; production stage manager, Diane DiVita; stage manager, Neil Krasnow; associate artistic director, Christopher Burney; production manager, Jeff Wild; general manager, Don-Scott Cooper. Presented by Second Stage Theater, Carole Rothman, artistic director. At the Second Stage Theater, 305 West 43rd Street, Manhattan; 212 246-4422. Through April 4. Running time: 2 hours. WITH: John Lithgow (Fitch) and Jennifer Ehle (Mrs. Fitch). 