With “Tully,” director Jason Reitman and writer Diablo Cody complete what amounts to a remarkable trilogy about the process of maturation women are destined to traverse in contemporary America. It began in 2007 with “Juno,” their remarkable comedy about teen pregnancy, which did not sidestep the subject’s darkest elements. Then they took on the issue of serious arrested development in 2011’s underappreciated “Young Adult,” with Charlize Theron as a young woman who can’t move on.

The actress now joins with them for a second time for a tale about a thirty-something housewife, already stressed out from dealing with two children, whose life is about to be further complicated by the arrival of a third. Taken together, the movies are an extraordinary exploration of the expectations placed on women in our society, but you can certainly appreciate “Tully” without having seen the others: it’s simply a first-rate comedy-drama about the joys and challenges of motherhood that both provokes and entertains.

Theron is Marlo, a New York suburban mom whose husband Drew (Ron Livingston) is a likable sort with good intentions, but is away at work all day, and while he helps around the house at night, he also spends a good deal of time unwinding with video games. That leaves Marlo, who is in the final stages of pregnancy, to deal with their two children, Sarah (Lia Frankland), a bright eight-year old who’s beginning to doubt herself, and Jonah (Asher Miles Fallica) a five-year old with behavioral problems suggesting he might fall on the autism spectrum and causing the principal at the kids’ school to suggest that he might need a special tutor or be better served elsewhere.

It’s in this situation that the already harried Marlo gives birth to Mia and finds herself overwhelmed. Her well-to-do brother Craig (Mark Duplass) offers assistance: he will pay for a night nanny to care for the infant so that Marlo can get some sleep, and his wife Elyse (Elaine Tan) testifies what a great help that can be with a newborn. Marlo initially resists, but, exhausted, she eventually agrees.

Enter Tully (Mackenzie Davis), a free-spirited twenty-six year old who proves a godsend, not only doing nanny duty but cleaning up the house and baking cupcakes, as well as, as we will see, performing other less conventional functions as well. With Tully providing almost preternatural support, Marlo is transformed into the nearly perfect wife and mother. The bond between the two grows ever stronger, until they decide to go out together one night to Marlo’s old neighborhood in the city, and what follows changes things radically for everyone—including the audience. Only some detours toward the fantastical raise doubts.

Theron, who hasn’t fared all that well since “Adult,” rebounds significantly here, giving a performance that captures the radical changes in Marlo’s personality with great skill. It’s also a fearless turn in purely physical terms, since after her pregnancy Marlo does not suddenly become svelte; she struggles to take off the excess weight, as her effort to keep up with younger joggers demonstrates. It isn’t the first time Theron has gone this route; “Monster” (2004) is the obvious predecessor, and it win her the Oscar. Perhaps she’ll be remembered by the Academy for this film as well.

“Tully” also represents a sharp turnaround for Reitman, whose last two pictures—“Labor Day” and “Men, Women & Children”—were misfires, and for Cody, whose “Ricki and the Flash” wasn’t among her best work. For Davis, on the other hand, it could be the breakthrough she needs, and deserves. She’s mostly bubbly and sweet, but also a bit tart when needed, and adds that touch of uncertainty about the nanny that’s required to keep one guessing about her real motives. She and Theron work splendidly together in the bargain.

The rest of the cast do able work. Livingston makes an amiable if none too capable schlub, and Duplass and Tan contribute sharp portraits of a privileged but not uncaring couple. Frankland and Fallica each have strong scenes as Marlo’s kids, and Gameela Wright contributes an incisive turn as their principal. The technical side of the movie—Eric Steelberg’s cinematography, Anastasia Masaro’s production design, Stefan Grube’s editing—aims at a slightly scruffy, ragged look, which is well suited to Marlo’s less than ideally organized life prior to Tully’s intervention.

This is a return to form for Reitman, Cody and Theron, as well as Davis’ decisive entry into the major leagues. “Tully” might get swamped by the popcorn fare surrounding it, but if so it would be a shame.