Jason Reitman’s “Tully” is, first and foremost, a warning not to have children. That’s not quite the intent of the film, and parents will surely find a relatable story about the ups and downs of raising a child, but childless viewers will have one primary takeaway: use birth control.

The brilliant Charlize Theron stars as Marlo, a very pregnant and very exhausted mother of two. Her husband (Ron Livingston) spends all of the day working, while Marlo, who is on maternity leave for the entirety of the film, takes care of the children. This early part of the film is engaging and very well executed, but feels very much like so many other movies about the monotony of parenthood.

There’s a particularly great montage scene that encompasses Marlo giving birth to their third child and taking care of it for the first few weeks, set to a rhythmic beat of car doors opening and slamming. It evokes something from an Edgar Wright movie, but drives home to the audience how slowly life moves for Marlo. Theron sells it perfectly, slowly deteriorating from exhaustion until her brother Craig (Mark Duplass — who looks confusingly similar to Ron Livingston) hires a night nanny to take care of the baby while she sleeps.

Structurally, this early part of the film is oddly similar to a crime movie, as Marlo initially has a personal issue with the idea of another woman taking care of her child, but caves when she realizes she can’t do it alone. But once the night nanny arrives, the whole movie makes a radical shift in pretty much every way.

Tully, the titular character played by Mackenzie Davis, is a sort of millennial Mary Poppins, but instead of existing to entertain children, she’s there to take care of Marlo. Tully helps the newborn sleep, cleans the house, bakes cupcakes and does basically everything Marlo wishes she had extra time to do.

Once Tully arrives, Marlo slowly comes back into her own. She’s more awake, she’s kinder and she starts wearing makeup again. Theron’s gradual change over the next few weeks in the movie is subtle but clear — you can literally see the color come back into her as she wears brighter colors and even has a different posture.

The highlight of the film is when Marlo and Tully are just talking to each other, as the two characters peel back each other’s layers. Tully, who at first seems like a one-dimensional fantasy of the perfect nanny, clearly cares for Marlo, and gets her to open up in a way not even her husband Drew can. The audience gets to know more about Marlo’s past in these scenes, but they never end up feeling like empty exposition and always reveal something more about who these people are. There’s a thematic richness in these conversations, as Marlo, reflecting on her younger years, sits directly beside someone living that early part of their life.

If “Tully” had ended after about 80 minutes, it would be one of the year’s best movies, easily rivaling “Annihilation” and “Paddington 2.” Unfortunately, as it approaches a conclusion, the movie takes a hard left turn, with a contrived plot turn that turns the subtext into bright, flashing neon letters. It throws the hard-earned subtlety right out the window, undoing what made the movie great in the first place.

Despite its misguided finale, “Tully” is a delightful modern take on Mary Poppins, bolstered by one of the best performances in Charlize Theron’s jam-packed catalogue of great performances.

“Tully”

• Rating: R
• Runtime: 93 minutes
• Score: 3.5/5 stars