Like his previous picture “Spy,” Paul Feig’s “A Simple Favor” is about a woman’s transformation, but while the Melinda McCarthy movie was upbeat and slapstick-funny, this one is darkly humorous and kind of nasty. That doesn’t mean it’s necessarily worse, just very different—though it is, in fact, not quite in the earlier picture’s league.

Based on a novel by Darcey Bell, the plot is about the curious friendship that develops between Stephanie Smothers (Anna Kendrick) and Emily Nelson (Blake Lively). Smothers is a widow, but an incessantly chipper mother to her little son Miles (Joshua Satine); she also hosts a video blog in which she dispenses advice to moms about food, creative household projects and ways to keep the children happy, and is such a volunteer at school that organizers have to ask her to slow down and the neighbors joke about her endless effervescence.

By contrast Emily is a sharp-tongued, impeccably stylish career woman—she works in what passes for haute couture, keeping the house designers in line. She’s married to handsome Sean Townsend (Henry Golding), a novelist and English lit professor at a local Connecticut university, and they have a son, Nicky (Ian Ho), who’s a classmate of Miles’. As a mother she might be termed laissez-faire, but she and Stephanie strike up an odd companionship as their kids become best friends, and before long Stephanie is happily picking Nicky up from school when Emily is busy, or arranging elaborate playdates for the tykes. The two women also share the cold martinis Emily favors while talking about themselves—during which Stephanie reveals some pretty unsettling details about the deaths of her husband (Eric Johnson) and her half-brother (Dustin Milligan) in a horrific car crash.

The first part of the movie, with fluttery Stephanie and acerbic Emily playing off one another, is good, snarky fun, but then the plot kicks in. Emily asks Stephanie to pick up Nicky one day, and hours pass without word from her—hours that stretch into days. Sean is in London tending to his sick mother, and rushes back; as Emily’s absence grows longer, Stephanie becomes his helpmate, sending tongues in their Connecticut burg wagging. The police, led by a savvy, jocular detective (Bashir Salahuddin), begin to suspect something nefarious.

Most importantly, Stephanie decides to investigate on her own, using her vlog to solicit not just sympathy for the grieving husband but information on Emily’s whereabouts. It turns out that the missing woman’s actions were in fact curious. It also becomes clear that her marriage with Sean was hardly as perfect as it seemed to Stephanie on the surface. Suggestions of infidelity and reports of large insurance policies are added to the mix. Finally Emily is found.

But the mystery doesn’t end there, which is where “A Simple Favor” threatens to veer off the rails into nutty-noir territory. Stephanie looks into Emily’s past and uncovers some very dark secrets. These, quite frankly, show that the screenplay is more interested in brash deviousness than even the remotest touch of plausibility, and in the process of explaining things it relies on one of the hoariest, most clichéd revelations of the genre. But the main emphasis is on how, in the process, Stephanie becomes quite a different woman—though one you might not find all that attractive.

And, of course, it’s the very ludicrousness of the explanation that’s meant to provide fizz, and it must be said that Feig and his cast bring enough catty malevolence to the final stages to keep “A Simple Favor” from becoming a drag. After all, noirs were never known for their subtlety or logic, and in that respect this is true to their spirit while adding sleek modern touches, like John Schwartzman’s vivid widescreen color images, which take advantage of the locations, Jefferson Sage’s luscious production design and Renee Ehrlich Kalfus’ witty costumes without looking for an instant like the shadow-infected black-and-white style of the old classics even as the script drops blatant references to many of them, sometimes by title.

The cast certainly throw themselves into that spirit, with Kendrick reprising her agreeably nerdy shtick and even dialing it up a notch, and Lively offering a hilariously shrewish performance that can turn mean on a dime as needed. Golding makes a convincingly befuddled hunk, and nifty, scene-stealing supporting moments come from Andrew Rannells, Kelly McCormack and Aparna Nancheria as a gaggle of neighbors who serve as a sort of Greek chorus; Salahuddin as that Columbo-like policeman; and Rupert Friend, as an arrogant designer Stephanie takes down a peg.

“A Simple Favor” is hardly the equal of the Coens’ neo-noir films, but thanks to savvy direction and very capable stars it brings sufficient smarts to the party to remain enjoyable even through a goofy final act. If you found pictures like Harold Becker’s “Malice” or Robert Zemeckis’ “What Lies Beneath” (2000) to your liking, this movie will provide a similar cup of bitter tea.