Some couples are a guessing game — you’ve gotta work to figure out why they’re together. But the central pair in HBO’s Divorce — Sarah Jessica Parker’s Frances and Thomas Haden Church’s Robert — are so patently wrong for each other that it’s hard to believe they ever got married in the first place. She’s a chic headhunter who walks around her Westchester mini-mansion in four-inch heels while realizing her dream of opening her own gallery; he’s a failing renovator who calls pooping “making a twosie” — and does so into plastic coffee cans. It’s not worth taking a side during their impending split, partly because the show doesn’t want us to and partly because neither character is written well enough to root for.

Creator Sharon Horgan (Catastrophe) and showrunner Paul Simms (NewsRadio) focus almost exclusively on the end of Frances and Robert’s relationship. But without knowing what the Lanvin bore and the Lands’ End boor had together, it’s difficult to appreciate what they’re losing by separating. That’s one of the several miscalculations that make Divorce such a middling letdown despite its sexual candor. It’s a somber dramedy that’s neither all that funny or gripping, about a couple that’s got a lot more money than personality. Even their tournament of one-upmanship feels floppy and limp. The show promises blood, but only gives its feuding opponents rubber swords to fight with.

Admittedly, last night’s premiere began with live-wire energy. Drunk and resentful at her 50th birthday party, Frances’s friend, Diane (Molly Shannon), brandishes a gun, waving it at her husband, Nick (Tracy Letts), then at her own head. Nick lands in the hospital — though not with a bullet wound — and Frances arrives at an epiphany: “I want to save my life while I still care about it.” For at least the next five episodes, that’s the most excitement the show ever sees.

Divorce doesn’t necessarily need sparks, but it does need to tell us who Frances and Robert are as people. What the series presents, though, are cardboard figurines from a paper-doll booklet called Separation Scenarios. The pair go through mediation, avoid telling their teenage children, attend couples therapy, lawyer up, and finally tell the kids — all with little sense of how it affects them. After the pilot’s reveal of her affair with a professor (Jemaine Clement), Frances expresses regret that she’s an adulterer. But we don’t really know how that new identity colors her self-portrait, despite gossipy scenes between Frances, Diane, and their third (and already divorced) pal, Dallas (Talia Balsam). A mental spinout seems inevitable after such a huge life decision, but Frances’s guilty blathering to her divorce attorney about how she never faked orgasms with her husband fails as both comedy and drama, because it’s never believable that someone as poised as she would ever fall that far.

The same goes for Robert, who flirts with casual misogyny post-separation and partners up with a woman-hating lawyer to extract alimony payments from his wife. Anxiety about change and the unearthing of buried resentments can lead many people to do things they never thought they’d do — and that’s a promising source of drama. But we don’t know enough about Frances and Robert to know what their dark sides look like, and the scenes that do explore their crueler impulses are so tonally off that we lose track of who they are, other than a married couple who pass each other in the hallway with middle fingers aimed at each other. It’s a vaguely funny image, but it’d be a lot more meaningful if Divorce actually gave us the opportunity to explore the layers of their mutual loathing.