At his best, director Jason Reitman crafts stories that are as acerbic as they are generous and tender (see: Juno, Up in the Air). Too much in either direction, though, usually results in Reitman producing saccharine, soggy dreck like Labor Day, or a jagged little pill like Young Adult (sidebar: Young Adult is a film I like quite a bit. But I can see why some people don’t, and I have no desire to revisit it.). Men without Women, neither cutting nor lovely, passed by like tumbleweed.

Tully is different; The film is challenging, and perhaps his best work since Up in the Air. It’s Reitman and frequent collaborator and screenwriter Diablo Cody operating at the peak of their prowess: he, humane and gentle, she, tartly concealing a wound. I didn’t immediately warm to this flick. Actually, I often alternated between exasperation and captivation – and a key plot twist at the end left a sour taste in my mouth, though for petty reasons. Nonetheless, something about it didn’t feel quite right. It took one observation from a friend afterward to allow for the film’s brilliance to bloom in my mind.

Regardless of what you think about the plot machinations, it’s all held together quite remarkably by Charlize Theron’s unvarnished performance as Marlo, a beleaguered suburban mother in the last days of her third pregnancy. Her little boy Jonah is a ‘quirky’ problem the school delicately/indelicately deals with. Her husband is a useless nice guy who too easily retreats into his video games. The poor woman can’t even buy a decaf without getting harangued. Her house is littered with broken toys and mushy microwaved food. She’s on the edge of a breakdown.

This part of Tully is Reitman and Cody firing off on all cylinders. In particular, the lethal contempt for a smug mother who warns Marlo about the dangers of microwaved food angrily radiates off the screen. But beneath the funny lines and awkward scenarios, there’s genuine fear and pain.

Enter the free-spirited night nanny: an all-of-26 Tully. She looks after the baby at night, so Marlo can get some desperately needed shut-eye. She’s a soothing and helpful presence, even going so far as to clean their house every night and lend a sympathetic ear to Marlo, who all but says out loud that she’s suffering from postpartum depression. As played by Mackenzie Davis, Tully’s like a hip millennial Marry Poppins. It all seems too good to be true. Their deepening connection hints at something that’s either eerie or profoundly healing.

And that’s when Tully gets a little goddamn strange. And by a little, I mean a lot. You’ll either be down with the final third or you’ll be violently thrown off the ride. I reiterate that it was a good thing I saw it with somebody to tell me that it was actually a good thing I was thrown off the ride, to stretch this already thin metaphor. I probably was too busy trying to figure out where it was all going, because there’s a long stretch in the middle where Tully appears drama-less, and you can't help but nervously wonder where it's all going. This is a fault on my part - it's not an ideal way to watch a movie. Best seen with the knowledge that Reitman and Cody know exactly what they’re doing.