This is a film predicated on that old adage that the grass is always greener.

The gorgeous Anna Kendrick plays Stephanie Smothers: A widowed, over-functioning mother who makes up for her lack of a life by running her son’s with peppy enthusiasm.

Stephanie also hosts a vlog (parlance for video blog) where she cooks child-friendly meals and reaches out to other lonely mums through cyberspace.

She’s an overbearing and even tragic figure who is ostracised by the other parents at her son’s school and lives on the income from her husband’s death insurance.

So when this overachieving and rather sickly sweet woman bumps into the chic and world-weary Emily Nelson (played by stunning Blake Lively) things look set to become infinitely more interesting.

The pair go back to the opulence of Emily’s house while their son’s have a playdate, knock back daytime martinis and get outrageously drunk.

Unleashed by gin and vermouth, Stephanie shares her deepest held secrets.

(They are black and her husband’s death - albeit tragic - is a bizarre and dark event. The film doesn’t ham it up enough, though.)

All this seems merely to amuse Emily who seems dangerously older, wiser and yet deeply dissatisfied by her seemingly perfect life.

She complains that money is tight and she has to pay all the bills with her high-powered job as a PR executive, while her husband struggles to write another book.

When Emily’s novelist husband Sean Townsend (Henry Golding) arrives home and passionately kisses his wife, Stephanie is green with envy.

To the isolated, single parent, Emily seems to have it all.

Emily draws on Stephanie’s overwrought parenting style for childcare on occasion, asking each time for “a simple favour” (where the story gets its name).

But one day Emily fails to return to pick up her son. She doesn’t answer her phone and no one seems to know where she is.

Emily’s husband Sean returns from London where his mother was in hospital.

A worried and fearful Stephanie befriends him and together they call the police.

Stephanie cooks meals for Sean and helps him look after his son.

The mystery deepens and gains traction online as Stephanie drops information about her “best” friend’s disappearance on her vlog.

When Emily’s body suddenly turns up in a lake, Stephanie is on hand to help with the funeral and the wake.

Numb with Emily’s sudden disappearance, Sean and Stephanie grow close, even intimate, as the police investigation swirls around them.

It’s not until Stephanie accepts Sean’s invitation to move into the house that things become disturbingly twisted.

I don’t want to spoil the film so I won’t say too much more about what happens next.

As the story unfolds - in all its unlikely fervour - it takes on a black, comedic tilt.

For me, the transition took hold just a little too late.

A fabulous soundtrack, with an opening Serge Gainsborough song Laisse tombe les filles dedicated to femme fatale imagery, hints at where the film could be going.

But it’s all lost in the execution, as if director Paul Feig could not decide whether it was a thriller, a schlocky murder mystery or a dark and twisty black comedy.

The best and most hilarious moment of the film is near the end and involves a character played by Andrew Rannells (of Girls fame) and a hybrid car.

It’s the kind of deadpan humour the film needs more of.

A Simple Favour is really beautiful to look at: stunning leading ladies; gorgeous garments and beautiful hedonistic shots dripping with good taste.

All that opulence left me wanting more of the ugly gristle that makes a solid black comedy really come to life.

I wanted Stephanie to be more unhinged. I wanted to Sean to be more malevolent.

Emily was perfectly psychopathic but her backstory was just given too much realism.

It’s a long film at 117 minutes but overall, an entertaining ride.

A pinch more malevolence and amping up the schlock factor would have made A Simple Favour really sing.