Dir: Jason Reitman; Starring: Charlize Theron, Mackenzie Davis, Ron Livingston, Mark Duplass, Elaine Tan. 15 cert, 96 mins.

It was a bounce of burnished hair that marked Rita Hayworth’s big entrance in Gilda, while Cyd Charisse stole into Singin’ in the Rain toes first. In the new film from Jason Reitman, Charlize Theron comes in with a bump.

The actress plays Marlo, an early-40s homemaker who is, we quickly gather, a mother first and foremost. In the film’s opening scene, the camera tracks her nine-months-pregnant belly as it makes its way across her son’s bedroom, only later pulling back to reveal the woman attached.

After the best part of a decade raising two children – one of whom demands more attention than she can humanly spare – Marlo has come to be defined by motherhood. While this hasn’t happened against her wishes, the grinding reality isn’t exactly in lock step with them either, and the impending arrival of child number three – unplanned, but officially a “blessing” – has her nerves wound triply tight.

What she needs is a younger, more energetic double who can shoulder some of her responsibilities – or failing that, just allow her a few nights’ unbroken sleep. What she needs – or rather who she needs – is Tully.

Tully (Mackenzie Davis) is the 26-year-old night nanny who appears on Marlo’s doorstep after coming highly recommended – and entirely paid for – by Marlo’s rich brother Craig (Mark Duplass). The film is self-aware enough to recognise the intensely middle-class-problems nature of its premise – “I know you think this is some bougie thing that only rich a______s do,” Craig says, after passing on Tully’s details.

But Theron, who gained three stone for the role, makes the fine detail of Marlo’s struggle so frazzlingly plausible that only a ghoul would begrudge her the help.

And what help. Despite Marlo’s early misgivings, Tully turns out to be something of a millennial Mary Poppins: whip-thin, wide-eyed, unnervingly earnest, and capable of soothing squalling infants with preternatural ease. She arrives while a trashy late-night reality show about gigolos blares from the television, and there is an unspoken suggestion that she, too, is providing a clandestine personal service.

The Fatal Attraction-esque overtones of a lithe young woman insinuating herself into a married couple’s life are toyed with so slyly that it’s hard to tell what kind of film you’re watching, beyond an intriguing one.

Tully Mackenzie Davis stars as Tully

Tully is Reitman’s third collaboration with the screenwriter Diablo Cody since 2007’s Juno, and their second after 2011’s superb Young Adult to star Theron. (In the interim Reitman hit the worst form of his career, so it’s a welcome reunion.) There is real, nerve-jangling fun to be had in watching the various cheering but also boundary-infringing ways in which Tully’s presence restores sanity, and then even joy, to Marlo’s life.

She and Drew are also parents to a mature and independently minded eight-year-old girl (Maddie Dixon-Poirier) and a kindergarten-aged boy (Asher Miles Fallica) who’s nothing of the sort, and whom friends and teachers have politely diagnosed as “quirky”.

That running joke is par for the course in Cody’s script, which in its best moments deliver both a paper cut and the lemon juice to make it worse in a single sharply turned line or exchange. (When a late-night conversation with Tully turns to Marlo’s sex life, she describes her postpartum body as looking like “a relief map of a war-torn country”.)

Yet as more information about Tully’s background creeps into the light, you realise a very bad decision has been made: in order to be “clever” – scare-quotes extremely necessary – the film sweeps away all of its hard-earned smartness, and the previously gripping uncertainty around the exact nature of Marlo and Tully’s connection is tidied up in a way that feels jarringly cheap.

Tully’s aim was obviously never thus, but you can’t help but wish the film had boldly ridden out its ambiguities Donnie Darko-style, because when it pins down the exact point it wants to make, it immediately feels like a smaller, less insightful picture.

Approach in cinemas with that proviso in mind, and you’ll at least be ably to enjoy a terrific Theron performance and some spikily well-observed mothering humour. Or you could wait for the more satisfying cut on DVD, during which, 15 minutes from the end, you quietly press eject.