Charlize Theron puts in an astonishing central performance in Tully, from director Jason Reitman, baring both ravaged body and soul as harried mother Marlo. Pushed to breaking point after the birth of her third child, a desperate Marlo accepts her brother's gift of night nanny Tully (Mackenzie Davis), who restores a sense of calm and offers Marlo a friendship that slowly grows in strength.

That nubile twentysomething Tully sweeps into Marlo's life bare-midriffed and perky suggests that this might be an entirely different film. Yet Diablo Cody's smart, perfectly observed screenplay sidesteps the cliché of peppy youth versus bitter experience to mine the sometimes funny, often-brutal truths of motherhood, ageing and the burdens of responsibility. While the teen protagonist of Cody's 2007 debut Juno (also directed by Reitman) was faced with the choice of whether or not to keep her baby, Marlo is up to her neck in parental pressure, going through the bleary-eyed motions because there is nothing else she can do.

Reitman handles the material with sensitivity and an unflinching eye for the detail of pregnancy, parenthood and female friendship that's by turns humorous and desperately moving. Throwaway lines about sugar, caffeine and screen-time jostle amongst more dramatic moments concerning Marlo's troubled son Jonah (Asher Miles Fallica), the loss of her individuality and the strength she draws from Tully's support. And if male characters are somewhat underdeveloped, that in itself makes its own point about the traditional approach to the division of parenting labour.

When the film takes a sharp turn, the comedy gives way to darker psychological excavation, and it's a testament to its robustness of character that it keeps the audience firmly in thrall. While there is plenty to laugh at, Tully reveals itself to be an authentic and achingly honest exploration of motherhood, an unpicking of life's rich tapestry and a reminder to appreciate what you have – even if, like the film itself, it becomes something entirely unexpected.

General release from Fri 4 May.