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Screenwriter Diablo Cody and director Jason Reitman made a strong impression with Juno, a comedy about a pregnant teenage girl who decides to give her baby up for adoption. Some of the dialogue has aged poorly, yet there is an observant core to the film since Cody and Reitman are shrewd about the boundaries and challenges of modern romantic relationships. Both Cody and Reitman have faltered since—Jennifer’s Body was an unmemorable horror film, while Labor Day was an outright catastrophe—but their latest collaboration, Tully, is a return to form. This intense, brooding dark comedy is a spiritual sequel to Juno, an unglamorous exploration of one woman’s exasperation over motherhood and how that leads to her mourning her youth.

Charlize Theron stars as Marlo, who is in her early forties and about to have her third child. She is starting maternity leave, and is quietly miserable. Perhaps she sees home as a prison, where she will soon be tethered to a newborn who needs constant care. Either way, this pregnancy does not have the anticipation or fear of her first: her husband Drew (Ron Livingston) sleeps through the delivery, and can barely muster excitement when the kid is born. Reitman handles the delivery sequence with wry observation: His camera glides over emergency rooms and hospital beds, framing Marlo on the edges and backgrounds so her delivery has all the drama of a routine outpatient procedure.

She takes the kid home, and struggles through those first weeks (her exhaustion is amplified by her toddler son, who has developmental issues). In a moment of desperation, she decides to hire a nanny that was suggested by her affluent brother Craig (Mark Duplass). Her name is Tully (Mackenzie Davis), and once she arrives, she ingratiates herself to the newborn almost immediately. At first, this interloper makes Marlo uncomfortable—Tully is young and pretty, with a bubbly personality—but her presence has the intended effect: Marlo catches up on much-needed sleep, and can spend her waking hours focused on her family. Soon Tully is more than just a nanny. She becomes Marlo’s confidant, and their relationship goes in some bizarre, unruly directions.

It is remarkable that Theron was in Atomic Blonde, a spy thriller with demanding stunts, just about a year ago. This role has none of that film’s kinetic fight scenes, and yet her performance here commands just as much attention. Along with Cody and Reitman, Theron makes zero apologies for her character. There are some moments where her behavior could be seen as exaggerated, even irresponsible, except Theron’s committed performance keeps us on Marlo’s level: After a meeting with her son’s principal, she knocks the baby carrier against a desk while the baby is still in it. This is not a sign of Marlo losing her mind, but her simmering frustration. Nearly every scene is from her point-of-view, even her long chats with Tully, and we slowly realize her funk is due to encroaching middle age and the loss of her identity. If everyone expects you to be a mother first, Tully suggests, then there is no room for the woman you were before.

Before Marlo has her third kid, Tully unfolds like a satire. No one looks her in the eyes, since they are drawn to her belly. It would almost be funny—Marlo has a cutting, caustic sense of humor—if it were not for the accompanying exhaustion and discomfort. But as Marlo and Tully get comfortable with one another, the satirical streak drifts away and the film shifts toward a tightly focused character study. Marlo’s life shrinks once she is on maternity leave, and newfound rest affords her a chance to reflect. She speaks wistfully about who she once was, and when Tully dishes about the kind of drama that only matters in your twenties, Marlo forgets her nostalgia and tries to impart some wisdom. Theron and Davis handle this material carefully, creating a sense of genuine spontaneity with characters who are delighted they can still feel any surprise.

Tully does not contain big revelations, since it is all about Marlo’s arc. There are enough little details that highlight the despair that peppers her life: Drew and her go through the motions of marriage, collectively raising their children as best they can, except Drew plays a video game—effectively abandoning their relationship—the first moment they have any privacy. Most films would focus on their marriage, with the husband/father getting equal attention. To Tully’s credit, we only see him how Marlo sees him, and where we leave their marriage depends entirely on whether they stop easy routines that lead toward bad behaviors (even if it’s mostly his fault). This is a film about mothers and motherhood, but through its stubborn empathy, even people who will never be parents have some idea of just how exhausting and unfair it can be.

Tully opens Friday at Landmark E Street Cinema.