Charlize Theron stars as Marlo in "Tully," by director Jason Reitman. Picture: Kimberly French, Focus Features

Star rating: 4/5. Rated R. Contains coarse language and some sexuality and nudity. 96 minutes.

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You could say that "Tully" is a familiar comedy-drama about a stressed-out mom in her 40s who, after her third child, finds physical support and emotional solace in the company of a 26-year-old child-care aide. 

There's the trope of the disengaged dad (a warm and shaggy Ron Livingston) and even a sight gag about stepping on stray Legos, barefoot.

But that synopsis does an injustice to the film.

Not because it's inaccurate, but because it doesn't begin to capture the cinematic sleight-of-hand by which this third collaboration between director Jason Reitman and screenwriter Diablo Cody ("Juno," "Young Adult") transforms itself from a garden-variety tale of harried parenthood into something stranger, more honest and even magical. How it accomplishes that trick must not be revealed; it involves one of filmdom's great bits of storytelling trickery. But that it does so, and in the service of a narrative that is at once prosaic in its specificity and marvelously poetic, should be celebrated, even sung from the rooftops.

Charlize Theron, never better, plays Marlo, an expectant mother of two young children who, after her second child (Asher Miles Fallica) is kicked out of school because the staff is unable to accommodate what administrators refer to as the youngster's "quirks," decides to hire a night nanny to help care for her newborn. Such a position, which combines the skills of an infant sleeping coach, lactation specialist and Mary Poppins, entails watching the baby so that Mommy can get her beauty rest, only waking her up - ever so gently - for feedings. If you get a good one, the night nanny might even make cupcakes and clean the house for you, while you're in Dreamland.

Such a gift is Tully (Mackenzie Davis), the almost impossibly wise, sexy, weird and slightly wild factotum who shows up - apparently paid for by Marlo's rich brother Craig (Mark Duplass) - and who quickly becomes not just the hired help but Marlo's best friend and confidante. Laid-back yet eager to please as portrayed by Davis, Tully makes for a wondrously levelheaded and insightful foil to Theron's middle-aged mother, offering Marlo not just a sounding board for her anxieties about aging and inadequacy - as a parent, as a partner and as a person - but also a devil's advocate.

Tully prods Marlo to reconnect with some part of herself that she believes she has lost. As Craig puts it, Marlo's like a match that's been "snuffed out."

Under Tully's care, she begins to burn a little bit brighter again.

Maybe too bright. Things come to a head one night when the relationship between Marlo and Tully, who seem to have developed some sort of symbiotic connection, crosses the line between the professional and, well, the reckless.

Relax. This is no horror film about a psychotic babysitter. But the dynamic between these two women suddenly shifts, dramatically. At the same time, Reitman and Cody maintain a firm grip on the steering wheel, guiding their story off-road not because they're lost, but because that's where they have decided to take us.

The choice is a bold and thrilling one, and it mostly works like a dream, except for one scene where the film's narrative gimmick stretches the audience's credulity a bit too much. That it works as well as it does is thanks to a screenplay that largely drops Cody's signature sarcasm - so often used to mask painful underlying secrets - for a voice that is much more raw and direct.

Not that "Tully" is cinéma vérité, by any means. Things are never exactly what they seem here - but there's a deeper, more authentic story Reitman and Cody are interested in telling, even when - maybe especially when - the film veers toward fantasy.

If "Tully" is a movie that cheats, even lies to us a little bit, it's to get at a more real and recognisable truth.

"Tully" hits South African cinemas on 11 May