There is something persistently fascinating about the trade-offs of longtime marriage: Everyone wants the intimacy, but almost everyone complains about the dimming of desire. Back in the late 1960s and ’70s --- when the opening salvos of the sexual revolution were just being heard --- open marriage, and what was quaintly called “wife swapping,” were proposed as solutions. Scandalous!

Today, such deals no longer seem quite so risqué, and yet, in a curious way people are more shocked by couples who decide to allow infidelity than by those who slide into secret affairs. In the last few months, there have been two movies on the subject (one, Open Marriage, a Lifetime melodrama; the other, Permission, a higher-toned indie currently on the film festival circuit) and a novel, NEXT YEAR, FOR SURE. And now comes another novel, THE ARRANGEMENT.

The author is novelist and TV writer Sarah Dunn, best known for “Spin City,” “Bunheads” (canceled too soon; I adored it) and, most recently, ABC’s “American Housewife.” Undoubtedly Dunn has her finger on the pop-culture pulse; her updated version of the perennial marital dilemma is thoroughly 21st century, while making it clear that some “arrangements” are still oddly taboo.

Lucy and Owen are recent transplants from the city to bucolic Beekman, New York, where they have chickens named Kiev, Marsala and Cacciatore; a big backyard; and a quaint downtown described as “Norman Rockwell-y.” One evening, friends from Brooklyn mention a couple who are experimenting with open marriage: a mutual agreement that it’s okay to sleep with other people. It plants a seed. Lucy and Owen have been married nine years, and raising their five-year-old son, Wyatt, who is on the autism spectrum, has been inconceivably tough. They make rules --- like “no falling in love,” “no leaving” and “no talking about it with each other” --- and set a time limit: six months. At first it’s a game. Then it’s a reality.

"Dunn is a lively, polished writer with a great ear for dialogue.... Given the author’s credentials, it’s not surprising that THE ARRANGEMENT is as sleek and smart as a high-caliber miniseries, and just as habit-forming..."

Does it work out? Surely you jest. THE ARRANGEMENT is a comedy --- admittedly, a dark one at times --- and the situation is ripe for everything from minor stumbles to disastrous missteps. Dunn is a lively, polished writer with a great ear for dialogue. She creates plenty of eccentric/trendy characters, like sex-changing Mr., then Mrs., Lowell, the kindergarten teacher, and his/her loyal wife; wisecracking, warmhearted Sunny Bang (the best “supporting” player in the book), who is mistaken regularly for the other two Asian-American women in Beekman; and billionaire Gordon Allen, who keeps bees for a tax break and made the mistake of not getting a signed prenup from his much younger wife. There are marvelously cinematic set pieces that show off Dunn’s screenwriting chops: a flashback to Lucy and Owen’s wedding, organized as a Renaissance fair --- complete with bagpipes and wimples --- by her professor father’s 38-year-old assistant-cum-grad student in medieval history; a surreal Blessing of the Animals that leaves one of their chickens dead meat; and the spectacle of local menfolk wearing Solidarity Skirts to express their sympathy for the Lowells.

But there are deeper themes here, too, mostly having to do with Lucy --- by far the most rootable-for character --- and her sense of having lost the juiciness and ardor of her younger self. Once embarked on The Arrangement, she begins eyeing men and realizes that for years she’d “been walking around with fifteen extra pounds on her, wearing bulky sweaters with things like foxes on them, obsessed with her son and his challenges, completely oblivious to the thick ever-present sexual haze that was in the air.” And when she has sex with a new man, it’s as if the dimmer switch she’d turned way, way down was back up, and “she was herself again for the first time in a very long time.” Whereas I felt that Dunn really got inside Lucy, Owen is more predictable and less likable; his escapades under The Arrangement have a slapsticky vibe. Yet he, too, winds up being sympathetic.

As does Wyatt, their son. When kids with disorders appear in fiction, they often become nothing but walking diseases. Wyatt isn’t romanticized --- he spits, tells Lucy he hates her and smears poop on the bathroom wall --- but he is smart and interesting, with weird obsessions (African black mambas, the Titanic, the Hindenburg, and other “tales of wide-scale death and destruction”). He has real individuality.

One criticism: Dunn populates her novel with a lot of other Beekman residents who, to my mind, detract from the main event: Lucy and Owen’s experiment. I suppose these less-than-ecstatic couples underline her points about the comforts and hazards of marriage, as do epigraphs preceding each chapter that feature an Esalen-type guru and author who, like a Greek chorus, throws out profundities on the limitations of long-term commitment. And then there is the ultimate irony of the Lowells’ marriage being the happiest in Beekman.

Given the author’s credentials, it’s not surprising that THE ARRANGEMENT is as sleek and smart as a high-caliber miniseries, and just as habit-forming (think “Big Little Lies,”another example of a closed-community melodrama involving sex and secrets). Although the ending, in my opinion, is too self-consciously plotted, there is a welcome ambiguity in the ultimate message. Don’t read this book if you’re smug about your marriage and want to stay that way, because it’s a strange combo of entertaining and subversive. You can expect to be both diverted and more than a little bit disturbed.