With “Tully,” their third collaboration, director Jason Reitman and screenwriter Diablo Cody are approaching a “Before Sunrise/Sunset/Midnight”-style series about the challenges of growing up and getting older.

It may not be the same set of characters, but “Juno,” “Young Adult,” and now “Tully” can be seen as a continuing story with smart, flawed female protagonists. The first was a pregnant teenager, then a floundering author trying to reclaim the life she thought she was going to have when she was in high school, and now a married mother of two, pregnant with a third, trying to connect with the person she once was when all of her time and attention was not taken up with obligations to others. Charlize Theron plays Marlo, the exhausted mother, and Mackenzie Davis plays the title character, a night nanny who turns out to be just what Marlo needs in ways that surprise her and us.

Marlo is exhausted and overwhelmed. Her second child is a son who is described by the school principal in the words that strike terror and a fierce defensiveness in the heart of a parent: “quirky” and “out of the box” — and then, finally, “not a good fit.” Every morning, with infinite patience and tenderness, Marlo gently brushes his whole body because she saw online that it might help him with sensory integration. When he has a meltdown because she isn’t parking in the usual spot, she is compassionate. But it is clear that she is giving so much to her family that she has lost some sense of herself and her own needs.

Marlo has a brother (Mark Duplass) who offers her a baby gift — the services of a night nanny, someone who comes at night to help new parents get some rest. Marlo and her husband (Ron Livingston) enjoy mocking her brother and his wife for being materialistic and bourgeois. It also makes them feel a bit superior and helps them ignore their envy at his financial stability. The idea of a “night nanny,” even paid for by someone else seems like just another mockable bougie pretension. But then Marlo has the baby as her husband’s job keeps him away from home and, even more exhausted and overwhelmed, she calls the number her brother gave her, and Tully (Mackenzie Davis) shows up like a hipster Mary Poppins.

She’s not just a baby whisperer; she’s a Marlo whisperer, too.  With patience and kindness, she listens attentively, and she very gently and supportively guides Marlo, to feel peaceful and cared for.  “Kiss the baby,” she says, as Marlo begins to trudge upstairs to bed. “She’ll be different in the morning.  We all will.”  Marlo comes down the next morning and the house is neat.  Tully even makes cupcakes.  She even presses Marlo about reconnecting to her husband.

Theron’s performance here is superb.  “Brave” when referring to an actress, usually means that she doesn’t look like a size zero teenager.  And Theron went for it here, gaining 50 pounds from her “Atomic Blonde” action star look, and she does indeed appear like a very pretty woman who has had three children.  But what is brave here is her extraordinary emotional authenticity, her vulnerability, and her wry humor.  When she tells the officious, though superficially kind school principal that the baby is a blessing, we can see three levels to that word.  Marlo is saying the word she thinks will ingratiate her with the principal who is trying to politely extricate her son from the school (those deadly words, “not a good fit”).  She is sarcastic (Cody loves sarcasm).  But, you know what? She really believes it, too.  Tully’s greatest contribution is reminding Marlo that all that is overwhelming her is what she once wished for.  This is her happy ever after ending and even if it’s going to be messy, there has to be a way to hold on to that before she ends up missing it.

Sharply but lovingly observed, clearly based on deeply lived experience, with lots of Cody’s shrewd wit and enormous compassion for its characters, “Tully” is as welcome in theaters as its title character is at Marlo’s door.

Parents should know that this film includes glimpses of pornography, explicit sexual references and nudity, very strong language, childbirth scene, drinking and drunkenness, and an auto accident with injuries.

Family discussion: Who is Tully? Why did she visit Marlo? Why are Marlo and her brother so different? What does “quirky” mean?

If you like this, try: “Juno,” by the same director and writer