A Simple Favor

A complicated martini.

Based on Darcey Bell’s novel, which must have been inspired by Gillian Flynn’s “Gone Girl,” “A Simple Favor” is a funny, dark-edged, lurid mystery-melodrama starring Anna Kendrick as a naive stay-at-home mom named Stephanie Smothers who turns detective when her hoity-toity best friend Emily Nelson (Blake Lively) goes missing. Eschewing the “females in peril” formula for one where the ladies control their own destinies, the film has director Paul Feig’s typically sunny, messy view of women, as seen in “Bridesmaids,” “Spy,” “The Heat,” and “Ghostbusters” (2016), but with less overt emphasis on comedy and more on MURDER (or the suspicion thereof).

Their young sons’ friendship notwithstanding, Stephanie and Emily are an unlikely pair of BFFs. Stephanie is a wholesome, chattery “soccer mom” type (and widow) with a swear jar at home; Emily, the high-powered head of P.R. for a world-famous fashion designer, has the phrase “Go f*** yourself” in her outgoing voice-mail message. Stephanie is intimidated by Emily, and the catty other moms and gay dads gossip about why a fabulous rich beauty like Emily would hang out with a timid loser like her. But though her educated mind has turned into “mommy brain” after several years of parenting, Stephanie is no dummy. She takes good lessons from the brash, sultry Emily, including valuable ones about how to deal with difficult men and when to say “I’m sorry” (about 90 percent less often than she currently does).

Emily, an inattentive mother on her best days, travels a lot for business and shares a too-lavish money-pit of a house with her husband, Sean (Henry Golding), with whom she has a tempestuously horny relationship. One day she asks Stephanie to pick up her kid after school while she attends to a work-related matter — no big deal; the boys play together all the time — and it’s a couple days before anyone realizes Emily has vanished-vanished, not just stopped answering her texts. The police get involved, suspecting Sean (it’s always the husband, right?) and flustering Stephanie — who feels guilty about her burgeoning friendship with her missing friend’s husband — with their questions.

Screenwriter Jessica Sharzer (who also adapted “Nerve”) gives Kendrick plenty to work with as a mousy doormat who must learn to stand up for herself, while Lively makes the most of her character’s eyebrow-raising yet undeniably effective life choices. We learn a great deal about both women’s pasts, and while the soapy plot gets pretty ludicrous, it avoids outright campiness, Feig and the cast striking just the right balance between taking things too seriously and not taking them seriously enough. The film runs smoothly, plot twists arriving at regularly scheduled intervals (40 minutes and 80 minutes, right on time), backed by a festive soundtrack of ’60s French pop. It spoils nothing to reveal that what we learn most about Emily and Stephanie is not to underestimate them.

Crooked Marquee

B (1 hr., 56 min.; R, a lot of harsh profanity, brief strong sensuality, some nude art.)