Despite its stars Blake Lively and Anna Kendrick and smart, women-led comedy director Paul Feig, A Simple Favour packs in too many tonal switches for its own good, says Simon Morris.
Review - Ever since Hitchcock - in fact, I'm sure long before Hitch, but he's the one you think of - the secret of a cool, smart thriller has been to regularly wrong-foot the audience.
It's called a mystery for a reason, and there's been a run of them recently, mostly written by and for women.
Gone Girl, The Girl on the Train, even the Fifty Shades of Grey books and films were based on the sudden switch - and now here's A Simple Favour based on a best-selling book by first-time novelist Darcey Bell.
We meet Stephanie - played by the endearingly eager to please Anna Kendrick (Pitch Perfect) - mother and "vlogger", producer of a video blog called "Hi Moms" offering other mothers handy tips and recipes.
But Stephanie has a darker story to share today.
Stephanie's little boy Miles has made friends with Nicky, the son of the cool, sophisticated Emily, played by cool, sophisticated Blake Lively (The Shallows), channeling every femme fatale from every film noir you've ever seen.
Emily overwhelms Stephanie, and in short order, starts taking advantage of her - particularly when it comes to looking after Nicky.
So Stephanie doesn't think anything of it when Emily asks her to mind him for a day or two - but this "simple favour" turns mysterious when Emily vanishes without a trace.
No one knows where she's gone, not even her husband Sean, played by the dashing Henry Golding, who's already established his old-fashioned movie-star credentials playing a similar character in Crazy Rich Asians.
So what's happened to Emily, and is she what we think she is? It's no spoiler to suggest that it's pretty clear she's not.
From the first moment she starts knocking back litres of martinis while in charge of an infant to ferreting out poor Stephanie's darkest secrets, Emily is born to be bad.
A Simple Favour is certainly a change of pace for director Paul Feig, best known for running a tight ship on female-skewed comedies, like Bridesmaids and Ghostbusters.
As the story continues to pile twist upon twist, Feig does a good job - particularly punctuating the film with Stephanie's video blog.
It's an elegant way to illustrate the old "unreliable narrator" gimmick. Not just 'bad' Emily but supposedly 'good' Stephanie have more than their share of secrets.
One of the occupational hazards of a thriller, I'm afraid, is the over-use of twists and reversals, however.
Fatally, A Simple Favour not only includes some pretty hard-to-swallow plot twists, but some near-fatal tonal ones too. From social satire to murder mystery to black melodrama to near-farce is a bridge or two too far.
The cast is great, but the film eventually turns too self-consciously cute for its own good.