What evil lurks behind the mommy vlogger with an "oopsie jar"?

Paul Feig's A Simple Favor is a suburban noir about two mothers with grade-school kids that veers into Gone Girl territory before turning more sinister still, and heading for the darker realm of Diabolique. It's an often lighthearted, sometimes creepy journey through the female stereotypes of the genre, with Anna Kendrick and Blake Lively as guides who both delight in and subvert traditional noir archetypes.

How good that sounds on paper. But Feig's A Simple Favor, adapted by Jessica Sharzer from Darcey Bell's 2017 novel, is a genre mash-up that fluctuates in such haphazard extremes that Feig never finds solid footing for his performers in his film's heightened, derivative reality.

Kendrick stars as Stephanie, a helicopter mom who gives online baking tutorials. Stephanie is mostly the kind of perky, nervous character that Kendrick excels at. Her eager, chipper disposition, though, doesn't jive with her character's recent past, which includes the death of her husband. She's the most go-go widow you've ever seen.

Through her son, she falls into the seductive orbit of Emily (Lively), a sexy, sophisticated, cynical mother who downs her vodka martinis in one gulp and greets her husband (Henry Golding, just as suave as he was in Crazy Rich Asians) by passionately making out. Watching their budding friendship, another parent (an underused, reliably terrific Andrew Rannells), remarks that the prim and cautious Stephanie is going to get "eaten alive" by Emily.

Emily has an air of mystery to even those closest to her. "She's like a beautiful ghost, never entirely there," says her husband. If there's a reason to see A Simple Favor, it's to watch Lively slink into a (seemingly) femme fatale role that's part Barbara Stanwyck, part Tyler Durden. Soon, hopefully, Lively will find a film that properly makes use of her full, glamorous powers.

But while Lively looks at home in the film's comic moments, Kendrick struggles in its dramatic ones. Those set in shortly after Emily asks Stephanie to pick up her kid from school-the titular favor-and never returns to pick her up. As things spin out of control, A Simple Favor teeters between camp and seriousness while the melodrama-incest, adultery, murder, twins!-gets nutty.

For the big tonal swings in A Simple Favor to work, the characters needed to be more plausibly grounded. Lively and Kendrick's early scenes ping-pong nicely with odd-couple chemistry, but A Simple Favor loses the thread, and never shakes the feeling of a rushed Gillian Flynn knockoff.

The most miscast here is Feig, one of our era's great comic directors (Bridesmaids, Spy, The Heat) who since Freaks and Geeks has justly earned a reputation for shepherding brilliant female protagonists. The material of A Simple Favor isn't quite up to his high standards, though. If he was here envisioning a Billy Wilder-like leap from farce to thriller, it can go down as a noble effort. Nobody's perfect.

A Simple Favor, a Lionsgate release, is rated R by the Motion Picture Association of America for sexual content and language throughout, some graphic nude images, drug use and violence. Running time: 116 minutes. Two stars out of four.