A widowed mommy vlogger with a sunny, go-getter disposition. A sophisticated, sharp-tongued workaholic mother who does not suffer fools gladly. Neither young woman may be quite what she seems, and a sudden disappearance causes a seismic shift in both their destinies. A jaunty mystery and tart black comedy in one, "A Simple Favor" keeps its twists coming, some more plausible than others but all working in the shadow of a wickedly naughty grin. Adapting from Darcey Bell's 2017 novel, director Paul Feig (2016's "Ghostbusters") and screenwriter Jessica Sharzer (2016's "Nerve") take the essence of their source material while making it their own. As for leads Anna Kendrick (2017's "Pitch Perfect 3") and Blake Lively (2016's "The Shallows"), they are terrific, bringing mordant zest and gravitas to their respective roles.

Stephanie Smothers (Anna Kendrick) is the kind of over-achieving mom who has to be talked down from volunteering for every parental duty on son Miles' (Joshua Satine) elementary classroom event sign-up sheet. Emily Nelson (Blake Lively), the chic director of public relations for fashion designer Dennis Nylon (Rupert Friend), is not. They are starkly different parents and people, but become unlikely friends after meeting during their children's afterschool playdate. When Emily must work late one day, she asks a simple favor of Stephanie: pick up son Nicky (Ian Ho) and watch him for the evening. Two days later, Emily still hasn't returned, and no one—not Emily's husband Sean Townsend (Henry Golding), in London caring for his ailing mother, and not Dennis Nylon himself—has any idea where she is. As the police get involved and an investigation into Emily's disappearance begins, Stephanie finds herself drawing closer to Sean while finding a personal flair for amateur sleuthing. In light of media attention over the case, her mommy vlog goes viral.

It would be easy to compare "A Simple Favor" to other recent films based on popular, female-driven literary thrillers—2014's "Gone Girl" and 2016's "The Girl on the Train" instantly come to mind—and, indeed, there are stylistic parallels in their whodunit, howdunit, and whydunit destinations. What keeps this particular picture fresh is its tricky, tongue-in-cheek tone, one which treats its story seriously but has a sense of fun and humor to offset the grimmer corners of the narrative. Sometimes it's funny, sometimes it wavers into camp, sometimes (particularly in the third act, which deviates significantly from the novel) it threatens to go a step too far into over-the-top goofiness, but it all, finally, comes together into a cohesive, bitingly entertaining whole.

Anna Kendrick (2017's "Pitch Perfect 3") and Blake Lively (2016's "The Shallows") are dynamite, the disparity between their parts adding to the appeal in seeing their barbed, emotionally candid interplay. Kendrick has fun toying with Stephanie's cheerful, girl-scout façade, the shameful secrets she's been holding onto finally coming to light once Lively's Emily is finished luring them out of her. Stephanie is a good person pulled into morally sticky situations after Emily goes missing, but she's not a pushover; a scene where she channels Emily's brutally honest energy while paying a visit to the ego-centric Dennis Nylon is delicious. There is another savvy moment, wonderfully played by Kendrick, where an increasingly paranoid Stephanie senses similarities with Henri-Georges Clouzot's classic 1955 thriller "Diabolique" and questions if she might be the victim of gaslighting.

Lively is every bit Kendrick's match as Emily, a woman who can be warm one second and prickly the next, the scope of her complicated character coming gradually to light once her haunting memory is all which remains. The supporting cast—Henry Golding (2018's "Crazy Rich Asians"), as Emily's college professor husband Sean; Andrew Rannells (2015's "The Intern"), as gossipy fellow parent Darren; Linda Cardellini (2015's "Avengers: Age of Ultron"), as hard-edged New York artist Diana; and Jean Smart (2016's "The Accountant"), as Emily's estranged mother Margaret—are appropriately colorful, but this is Kendrick and Lively's show through and through.

"A Simple Favor" is a slick, visually dynamic true-crime treat, cinematographer John Schwartzman's (2018's "Fifty Shades Freed") lensing as glossy and off-center as Stephanie and Emily's lives. There are also tinges of whimsy (the umbrella blowing randomly across a rainy street as Emily makes her grand, slow-motion entrance is an amusing touch), turning what could have potentially been a semi-derivative thriller into a succulently dark suburban fable. Cunningly structured, the film is most of all a showcase for two memorable lead characters, both complex women and both so much more than mothers, who learn in the most forebodingly serendipitous manner possible what they are made of. "A Simple Favor" isn't so simple at all, and that's the key to its mischievously layered pleasures.