'Tully' review: Charlize Theron shines in this unpredictable but moving comedy-drama

“Tully” is a movie that can be easily described but not so easily explained. It's about a middle-aged mom who turns to a younger nanny for help, but that deceptively simple story takes on a range of unpredictable moods and shadings: wryly funny, claustrophobic, emotionally shattering. It's never quite clear where “Tully” is heading until, in a revelatory moment, it gets there.

Initially, “Tully” seems like another episode of Hipsters in Crisis: Marlo (Charlize Theron), a onetime Williamsburg wild thing, has just given birth to her third child with Drew (Ron Livingston), a great guy who's solid as a rock, and about as interesting. As Marlo's life becomes an endless series of needy cries, swollen breasts and sleepless nights, she breaks down and allows her wealthy brother, Craig (Mark Duplass), to pay for a night nanny.

“Tully” shifts in mood when this moonlight Mary Poppins — the Tully of the title — shows up at Marlo's door. She's an immediately riveting presence, played to perfection by Mackenzie Davis (AMC's “Halt and Catch Fire”), whose eyes glisten with kindness, generosity and, perhaps, a touch of mania. As Marlo dozes through a blessedly uninterrupted night, Tully soothes the baby, cleans the house and even bakes cupcakes. “I'm like Saudi Arabia,” she says brightly. “I have an energy surplus.”

That's a great line, one of many that glimmer throughout “Tully,” which was written by Diablo Cody (the script was inspired by the birth of her third child) and directed by Jason Reitman. Those filmmakers and their star, Theron, also gave us the dark character-study “Young Adult,” and “Tully” feels like a companion piece. It's another story about a woman on the edge, and in its way even darker. Marlo, physically wrung out by childbirth — Theron gained a visible 50 pounds for the role — regards Tully's flat midriff and carefree attitude with a complicated mix of fondness, bitterness and suspicion.

Why is this lovely young stranger so understanding, so insightful, so presumptuously intimate, when it comes to Marlo's needs? That question won't be answered until the end, when everything in “Tully” falls perfectly and heartbreakingly into place.