Grade: 3.0/5.0

Life always goes on. That’s the adage of the works of Jason Reitman, the director who hit it big in the late-2000s with “Juno” and “Up in the Air” but has since had trouble reworking his penchant for collapsing perennially perturbed lifestyles and rebuilding them.

But armed with a take-no-prisoners performance from Charlize Theron, “Tully,” Reitman’s third collaboration with screenwriter Diablo Cody, represents a step forward. Tactful until it isn’t, the film is decidedly slight but not without its strengths as a candid character study.

Marlo (Theron) is years into her decision to settle down with her husband, Drew (Ron Livingston), who’s stable, affable and more than a little boring. Raising a precocious 8-year-old girl, a younger son with undiagnosed hypersensitive symptoms and an unplanned baby about to pop out of the oven, Marlo lives her day-to-day overcome with exhaustion from juggling her relentless line of chores as a mother.

In an act of reluctant desperation, she takes on her brother Craig’s (Mark Duplass) offer to hire her a night nanny, someone to take care of her newborn child in the dead of night. But Tully (Mackenzie Davis) swiftly becomes a therapeutic presence to Marlo.

From its perversely blasé delivery sequence, “Tully” demonstrates an acute understanding of how exhaustion can come to outpace guilt and make self-awareness feel like a privilege. One of the most bitter laughs comes early on, when an onlooker warns a pregnant Marlo that decaf contains trace amounts of caffeine. Marlo, looking as if she’s about to collapse, buys the coffee anyway.

Having built a career out of playing durable under bruising stakes, Theron truly graduated into the role of Marlo. Beneath her openly displayed, stretched-out baby weight and baggy eyes, you can see the flinty young woman she once was and how the demands of motherhood have evicted that person from her body.

Though the film is too compact to spend much time on the details of domestic life, the drama is ultimately driven by Marlo’s memories of youth that Tully’s presence dredges up. Fatigued by the present, it’s Marlo’s unspoken past and the rediscovery of the person she is that make “Tully” such a colorful star vehicle.

Mackenzie Davis gives an uncannily all-knowing performance as Tully, rife with eager stares and carried by an unflappable, pepped-up attitude. Her sex-positive, ostentatiously young Mary Poppins is here to save the mother, not the children. A seemingly impossible embodiment of an ideal of youth, the nanny provides a window for Marlo to gaze upon her past self. Theron’s largely unspoken mixture of resentment and gratitude toward this untethered woman provides the film’s dramatic bedrock.

Punctuating this gradual emergence from the broadly characterized middle-class domestic torture of foot-stabbing Lego bricks and high-pitched screams are brief, fantastical sequences of Marlo envisioning mermaids swimming to and fro. These unimaginative plays at subjective fantasy are little more than irritating interruptions of the film’s authentic humdrum rhythm. They remain disposable until the film’s botched endgame.

The reorienting efforts of the final minutes of “Tully” elicit more puzzlement over the motivation behind them than catharsis for those involved. Though Cody’s screenplay lays out the breadcrumbs for the sleight of hand, Reitman’s laidback, overwhelmingly frank direction fails to synthesize the thread of Marlo’s emotional fantasy with the carefully curated ordinariness of her lifestyle. The rug-pull comes across as disingenuous and unearned within Reitman’s sadistically unstylized patina.

As defeating as its self-consciousness comes to be, “Tully” works well enough on a moment-by-moment basis. The film shows an admirably mature indifference toward whether or not the audience likes its characters, instead dedicating itself to depicting how the elliptical labors of parenting can erode one’s personality.

Reitman and Cody possess the dignity to paint this ordinary world with unglamorous colors. At the end of the film, Marlo watches “Gigolos” downstairs with a plate of microwave nachos away from her wet blanket of a husband. They wake up the next day. Are they happy? Life goes on.

‘Tully’ is now playing at California Theatres.