“Tully” is an uncommonly intelligent, amusing and honest portrait of motherhood, thorns and all. It is the third and best alternative universe created by director Jason Reitman and screenwriter Diablo Cody, who collaborated to give us the quick-witted “Juno” and melancholy “Young Adult.” With Charlize Theron returning for her second lead role with the team, the new film hits the trifecta.

The movie is almost all relationships, with hardly any plot, and all the richer because of it. The script makes its points economically and winningly. We learn that Marlo manages human resources for a company that makes energy bars, an odd job for a stressed woman with minimal get-up-and-go and few people to count on as a social safety net.

Motherhood is a world where everyone wants something from weary, hollow-eyed Marlo, even before her delivery date. The issues in focus are remarkable for a film largely about childbearing, a subject usually filmed in tones that are all sugary sentiment or maudlin but not moving. Here, openly or implicitly, every woman Marlo meets seems to feel superior to her.

A rail-thin college girlfriend examines pumpkin-shaped Marlo and offers a halfhearted invitation to reconnect, before roaring away on a snazzy scooter. A counselor at her son’s parochial school suggests that the “quirky” boy needs a full-time one-on-one mentor — which would have to be financed through her family’s limited budget — or a transfer to a more appropriate kindergarten. A fussbudget customer at the coffee shop gives Marlo a patronizing look for ordering a decaf. It still contains trace amounts of caffeine. Won’t she think of the unborn child? Nice mothering!

“Tully” can best be summarized as an adult coming-of-age story. From the beginning, Marlo is in a solidly committed marriage to Drew (Ron Livingston), a decent clod with boundless love for her and their children but clumsy marriage and fatherhood skills. She is comfortable enough with her gruff, wildly successful brother Craig (Mark Duplass) and his not-at-all arrogant and snobbish trophy wife Elyse (Elaine Tan), who proudly announces that at the upcoming school pageant, their little daughter will perform Pilates.

Partly because none of them could effectively grant her the attention and the help that a new mother requires, and partly because she doesn’t want to appear needy, Marlo doesn’t ask for help with her imminent third child.

After the birth, as if in a miracle, help arrives late one evening when Tully (Mackenzie Davis) knocks on Marlo’s front door. Brimming with the joy and enthusiasm of mid-20s youth, Tully comes from a “night nanny” firm, bankrolled for a few weeks of service by Craig.

With skills that would make Mary Poppins envious, Tully takes the whole sleeping household under her wing. She gives Marlo lifesaving help with middle of the night feedings, a sympathetic soul to share her thoughts with and enough arcane facts about whales, cell biology and 17th-century English diarists to fill Wikipedia. And as for rebooting the shriveled conjugal connection between Marlo and Drew, Tully’s got some ideas along that line, too.

“Tully” concerns a mature, well adjusted, even brave woman’s response to the tender daydreams, acidic nightmares and erratic mood swings that modern postpartum life can impose, pulling us into its alternate universe. The film’s artistic license earns our poetic faith. See it.