Aside from very rare exceptions, it is almost axiomatic that a comedy that closes with the birth of a baby to signify that, after all the plot complications, all is well again is a loser. That’s certainly the case with Lake Bell’s second feature, a wispy, feeble sitcom-quality ensemble piece about, if you can believe it, the robustness of the marriage bond—in an age when divorce has become the order of the day.

The catalyst of “I Do…Until I Don’t” is Vivian Prudeck (Dolly Wells), a snooty British documentarian whose last film was “Teenage Jungle”—you get the idea. Her new project is based on her view that the concept of permanent marriage is no longer valid, if it ever was, and she’s come to Florida to enlist several couples whose marriages she will follow as they unravel.

She first latches onto Alice (Bell), whose union with Noah (Ed Helms) is happy enough on the surface—they’re trying to have a kid, which introduces the usual stuff about having sex when the apps indicate the time is right—but whose financial situation is precarious. Alice, a shy, recessive type, gave up her dream of becoming an artist to help Noah run the store he inherited from his dad—a blinds shop, of all things. Understandably, it’s going under, and they may have to declare bankruptcy. For some reason she’s a big fan of Vivian and worms her way into the project, though she doesn’t tell her husband it’s a non-paying gig and so—in one of the script’s worst bits—she takes a job in a sex shop in the hope of making some cash she can pass off as their salary.

The involvement of Alice will lead her free-spirited sister Fanny (Amber Heard) into the mix, although she’s not really married to her equally hippy-ish boyfriend Zander (Wyatt Cenac). They’re devoted to one another, even if Zander shows a jealous streak when a drifter named Egon (Chace Crawford) shows too much interest in Fanny.

Then there’s the third couple, realtor Cybil (Mary Steenburgen) and her semi-retired husband Harvey (Paul Reiser). His obsession with a newly-bought motorcycle irks her to no end, and their conversations consist of perpetually sniping at one another. Their marriage is further tested when daughter Milly (Hannah Friedman) shows up pregnant by a boyfriend whom they presume will be unsuitable. They’re drawn into the orbit of the other couples when Harvey decides to make use of an introductory couple at the aforementioned sex shop, where Alice is assigned to give him a massage.

The cast is an able one—vets Steenburgen and Reiser are particularly engaging, though their younger colleagues are mostly ingratiating (apart from Wells, whose stridency is insufferable). But Bell’s script, crammed with idiotic situations and lame dialogue (including long interview segments with the couples that are D.O.A.), offers them all virtually nothing to work with. Nor does her direction help, being so lackadaisical as to seem nonexistent. On the technical side, the movie looks a bit impoverished, with cinematography (by Wyatt Garfield) that’s utterly bland and a score (by Dexter Story) that’s irritatingly perky.

It all ends in a scene set at Vivian’s big outdoor project party where—surprise, surprise!—the strength of marriage is reaffirmed, followed by what makes it all worthwhile, that supposedly madcap moment when Milly’s water breaks. The desperation couldn’t be more apparent.

The original title of “I Do…Until I Don’t” was “What’s The Point?” You can understand the reason for the change, but somehow Bell’s initial choice seems the more appropriate one. Maybe she can recover the modest mojo of her first movie, “In a World…,” next time; in this case, what the ellipsis appears to indicate are the humor and heart Bell’s omitted this time around.