Marlo (Charlize Theron — Atomic Blonde) is not well. With only days until the birth of her unplanned third child, tested by the antics of her autistic second, Jonah — politely deemed “quirky” by the school principal — and receiving no help from her workaholic, unresponsive husband, Drew (Ron Livingston — Office Space), she’s already exhausted.

When her wealthy brother, Craig (Mark Duplass — Safety Not Guaranteed), whose privileged life is far removed from Marlo’s, gifts her a night nanny to care for the newborn — allowing Marlo vital time for herself — she is initially appalled. But when Jonah’s increasingly disruptive behaviour at school causes his dismissal, and the endless daily needs of baby Mia become an ordeal, Marlo implodes.

Enter Tully (Mackenzie Davis — Bladerunner 2049) — compassionate, thoughtful, mischievous, and with a wisdom seemingly beyond her years. She instantly takes charge, not only of the child but of the family, gleefully cleaning the house and baking cupcakes in the night, allowing Marlo to concentrate on the mother she needs to be, and to find the woman she once dreamed of being before slaving herself to family.

But Tully is almost too good to be true and she is not all she seems.

Academy Award-winning screenwriter, Diablo Cody (Juno) has written a sensitive two-hander with Tully, based on her own tribulations of motherhood. Marlo and Tully are a unique pair, and she’s blessed with two enormously likeable leads in Theron and Davis.

Theron has never shied from portraying unglamourous characters. Marlo is unrelentingly normal, heaving herself from baby feed to breakfast to school drop-off, to placatory meeting with the school principal, to a day on the couch watching reality TV, to school pickup to dinner to bed — and repeat. Her exhaustion is palpable.

Theron is so effective as the drained, numbed Marlo — where even getting dressed is a chore — that, when the vivacious Tully arrives, we actually give a sigh of relief. As she says to Drew, “I feel I can see in colour again”.

Mackenzie Davis is a delight, bringing energy to the film with her inquisitive, confident, twinkle-eyed Tully. We are immediately reassured by her practicalities with Mia, and yet, as time goes on, questionable cracks begin to appear in her character that allow for some suspicion.

It is through Tully that Marlo begins to fondly relive the passions of her youth, and simultaneously be revived in the eyes of others. Her brother’s gift is not entirely selfless, as he tells his wife Elyse (Elaine Tan — Inherent Vice), “I want my sister back”.

Yet Marlo’s youth is nostalgic and not something she can return to. In a heartening moment, Tully argues for the beauty of Marlo’s normal life: every day she rises and takes care of her family, which may not be the ideal she wished for, but despite Marlo’s fright at not being able fulfill her children’s lives and take charge of her own, it is meaningful.

This is pure storytelling, and Jason Reitman (Juno, Up in the Air) simply lets the characters speak. His direction is unfussy and naturalistic, but smart — especially in the fast cutting which allows us to experience the repetition of Marlo’s days, our nerves tightening in parallel, readying for her implosion. The darker themes of mental illness and post-natal depression are also subtly portrayed — real but not overwhelming.

Cody’s script brims with natural humour and tenderness. It also touches on that age-old question of which everyone has an opinion: how a mother should bring up her child.

The snobby Elyse gifts her children with an expensive karaoke machine and employs a qualified vegan nanny who presides over the children’s meals at a separate table. When told that Marlo doesn’t want the night nanny, she snaps, “Now she’s judging our choices?” To Marlo, the concept of a night nanny is one of privilege, and the suggestion that a mother should give over her newborn to a stranger lessens maternal responsibility.

But Tully is so empathetic that we cannot help but feel Marlo’s precious “time out” is not selfish but a necessary revivification. The greatest pathos comes when we realise who Tully is and what she means to Marlo.

At times, Tully is uncomfortable, and the poignant ending is hard-won. Yet it is down-to-earth, gently emotional, and has lots of heart. It features a couple of beautiful complementary performances from Theron and Davis, and quietly but effectively packs a punch.

What do you think about the idea of a night nanny?

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