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“Mom, what’s wrong with your body?” cries Sarah (Lia Frankland) as her mother, Marlo (Charlize Theron), flops into a chair at the breakfast table. Marlo just had her third kid and, after another sleepless night, admittedly looks a little worse for wear—adding to that, she’s opted to go sans shirt for this particular family meal. Tully, Jason Reitman’s new film about the trials of baby-rearing, doesn’t shy away from the physical toll of having a new kid in the house. But it’s less worried about Marlo’s body than it is about her mind.

Tully was scripted by the Oscar-winning screenwriter Diablo Cody (who collaborated with Reitman on Juno and Young Adult); she’s said the film touches on her own experience with postpartum depression after the birth of her third child. In its first act, the movie presents that situation with stark simplicity. Nothing’s outwardly wrong—Marlo’s new child is healthy, and her husband, Drew (Ron Livingston), is supportive, if a bit distant—but Marlo feels isolated even when she’s around other people, wandering around in a daze, performing her parenting duties on autopilot, and generally fraying at the edges. That is, until Tully shows up.

Tully (Mackenzie Davis) is a night nanny hired by Marlo’s rich brother Craig (Mark Duplass); she’s a sprightly 20-something who arrives in the evening to watch over the baby so that Marlo can sleep. Beyond that, though, she’s a genuine breath of fresh air, a person unbound by all of the commitments—parenthood, matrimony, a dull career—that clearly weigh on Marlo. Tully is the title character of the film, and she’s the spark of energy that makes it go, but she’s also a puzzle the movie spends its whole running time trying to solve. The answer (which is provided in the movie’s closing minutes) may not work for everyone, though I’ll say this: It’s undeniably daring stuff.

If Tully has a problem, it’s how frustratingly opaque the mother at its center feels most of the time, but that’s a feature built in to Cody’s script. Marlo is present in practically every scene in the movie, and yet she so often feels unreachable, even when crises present themselves (many of them revolving around her second kid, Jonah, who is struggling to fit in at school). The only time she really comes alive is when Tully is around, and those scenes have their own dreamlike quality to them.

Tully is endlessly open and nurturing without seeming possessive, helpful without seeming controlling. She’s the kind of person who walks into your home and immediately opens the fridge to poke around for snacks, and she’s always ready with some specific strange factoid or oddball news story. She should be annoying, but somehow isn’t, instead coming off as a Millennial Mary Poppins who always has the right answer but never brags about it. She is, essentially, Cody’s idea of a perfect caregiver, a sort of mystical mental-health nymph summoned from another dimension to guide Marlo through her baby’s infancy.

The movie continually suggests Tully’s otherworldly nature, too, cutting to dreamlike images of mermaids swimming, and, for most of the film’s running time, containing the Tully–Marlo scenes to their specific nighttime appointments. Nobody else ever sees Tully, but then again, not many other people are in Marlo’s life right now aside from her family, and even they sometimes struggle to acknowledge her as a human being. Then, in Tully’s final act, things start to get weirder, and the film lobs more than one surprising twist at the viewer in rapid succession. It’s spellbinding, at times tense, and certainly worth having animated conversations about as you exit the theater. But I’m not sure it all works.

Much of what does work is owed to Theron and Davis’s incredible performances. Theron, who’s long been one of Hollywood’s most impressive physical actors (in projects as varied as Monster, Mad Max: Fury Road, and Reitman’s Young Adult), sells so much of Marlo’s state of mind through the way she walks and holds herself; as Tully enters her life and begins to help her out, Theron communicates the relief effortlessly. Davis, who long ago demonstrated her versatility in projects like Halt and Catch Fire, Black Mirror, and Always Shine, threads the needle on a character that should either be infuriating or preposterous (or both!), making Tully enchanting and mysterious.

The film’s somewhat shocking conclusion (which I won’t spoil), at first, felt like one too many twists for me—a hat on a hat, an explanation beyond what I felt I needed as a viewer. As a portrayal of postpartum depression, Tully is a success—simplistic at times, but an admirable gut-punch nonetheless. The ending adds some complex layers on top of that portrayal, though, and left me with questions I couldn’t quite as satisfactorily answer afterward. But the more distance I’ve gotten from Tully, the more I’m convinced that’s the point—Cody doesn’t want the viewer to leave the film thinking they understand just what Marlo is going through. Life, particularly this sometimes-trying part of it, isn’t easily solved, and neither, by design, is Tully.

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