Marlo is exhausted. She’s not only about to burst with her third child, but her youngest might be kicked out of kindergarten, her oldest is reaching that age of budding insecurities, and her husband is ... busy playing Call of Duty or something. Being a mom is supposed to be wondrous and celebratory, right? In Tully, it’s an unrelenting nightmare of no sleep, endless high-pitched screaming, frozen pizza dinners, and the complete deprivation of all joy and serenity. In Diablo Cody’s third collaboration with director Jason Reitman following Juno and Young Adult, and the duo’s second with Charlize Theron, the screenwriter isn’t afraid to plunge into the bleaker depths of motherhood.

I don’t have children, and Tully might be the most convincing argument to keep it that way (or, at least, a warning for couples expecting an unplanned third kid). Cody and Reitman display the horror story of being a mother of three with a newborn, most notably, a mother with an absent-but-well-meaning husband (Ron Livingston, in an appropriately minimal appearance).

Based on the reactions of other critics with children, and the fact that Tully arose out of Cody’s experiences after child number three, I trust that what we see is fairly accurate. A normal morning of school drop-off turns to total chaos when Marlo’s (Theron) five-year-old Jonah and eight-year-old Sarah launch into curdling screams in the backseat. When the baby Mia finally arrives, the birth itself is a bore and a breeze; the real pain in the ass is a nurse forcing Marlo to pee afterwards. And after a meeting with the private school principal goes awry – Jonah, alluded to being on the autism spectrum, is too “quirky” for his teacher to handle – a ticked off Marlo storms out, accidentally smacking the baby carrier on her way out. Cue endless infant screaming.

Reitman and Cody condense the hellscape of Marlo’s life (and what I assume it’s like to raise a newborn) down to one incredible montage, turning daily parenting routines into a descent towards insanity: baby wipes, diaper pail, breast-pump, nail trimming, onesie, baby wipes, diaper pail… and on and on and on. You’ll go from laughing to flinching with anxiety; by the end of the montage, you could’ve convinced me I was watching a horror movie.

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But the third Cody-Reitman project isn’t just about the rough patches of raising kids; it’s more interested in exposing the uglier realities of motherhood and aging that few, if any, movies have touched on. If Juno is about growing up too quickly, and Young Adult about refusing to grow up at all, Tully, a lovely conclusion to this loose trilogy about women, is about what happens when you’ve done all the growing, lost yourself in the process, and can’t figure out where to go next. It explores the expectation that parents ought to sacrifice their own lives for the life they’re bringing into the world; once you’ve given your all, who’ll be there to take care of you? After the birth of Mia, Marlo is drowning in postpartum depression, so much that her only respite comes in ethereal dreams of a beautiful swimming mermaid. That is, until her brother-in-law Craig (Mark Duplass) arrives with the ultimate gift: a night nurse.

At first Marlo rejects the offer, opposed to letting a stranger care for her baby at night. But nearing a breaking point, she eventually makes the call. Enter Tully: a magnetic Mackenzie Davis sporting a crop-top tee and faded high-waisted jeans. She’s the ultimate cool girl, a manic pixie dream nanny (and this is one of the rare occasions where that cliche actually works perfectly) who says weird things about DNA and space – “You’re like a book of fun facts for fourth graders,” Marlo muses. Marlo is, understandably, a little weirded out by this hippie alien Mary Poppins, a concoction of everything she’s not: Young, fit, energized, has a social life, multiple sex partners, can eat whatever she wants, and has the maternal touch. With Tully, Marlo finally gets her first night of rest, and the two quickly become good friends.

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Theron and Davis are the heart and soul of Tully, a pair whose onscreen chemistry flickers with a magnificent glow. Watching their scenes together, late-night chats about exes – Marlo’s bisexuality is alluded to early on after a run-in with an ex-girlfriend – sex, aging, and motherhood, feels like being wrapped in a warm, cozy blanket; it’s a massive relief to finally see Marlo smiling and laughing for the first time. Theron and Davis toss Cody’s fantastic dialogue back and forth with a natural charm and ease. I only hope more filmmakers will have the smarts to case these two in more movies together.

Davis may play the character who rescues Marlo from the quicksand of motherhood, but this is fully Theron’s vehicle. She commits every fiber of her being to Marlo, and I’m not even talking about the 50 pounds she gained for the role. You can feel the weight of the exhaustion radiating out of her slumped over shoulders, her watery red eyes, and her oft-tilted head; who has the time for good posture? She seamlessly flips to the other end of the spectrum when Marlo is renewed with a burst of life, belting out “Call Me Maybe” with her daughter during karaoke. It’s a performance that belongs in the top tier of Theron’s career, alongside Monster, Mad Max, and Young Adult.

In the weeks since seeing Tully, I’ve wrestled over whether it’s a purely great film, or a great one with flaws, all because of its risky ending. There’s a twist that brings deeper insight and originality to the story, but it’s one Cody and Reitman don’t land as gracefully or sharply as they could have. It ultimately leads to a too-tidy conclusion that left me unsatisfied and a bit bummed out. That said, the first three quarters of Tully are pure magic, a darkly comedic and earnest ode to the woes of motherhood. It’s only too perfect that after some release date shuffling, its arriving just a week shy of Mother’s Day. Book your babysitter now.