In Tully, Jason Reitman’s third comedy with screenwriter Diablo Cody after the one-two gems of Juno and Young Adult, everything about parenthood is a lie. Marlo (Charlize Theron, terrific) is days away from giving birth to her third child, and she tells everyone things are great. “Such a blessing,” she mutters, almost reflectively—isn’t that what she’s supposed to say? Her husband Drew (Ron Livingston) is totally helpful, her special-needs son Jonah (Asher Miles Fallica) is progressing just fine, and as for the postpartum depression she suffered after Jonah’s birth, Marlo doesn’t mention it at all. In truth, Drew plays video games every night, Jonah is close to getting kicked out of his private school kindergarten, and Marlo’s wealthy brother Craig (Mark Duplass) correctly fears she’s speeding toward another breakdown.

Marlo is carrying a lot of weight – figuratively and literally. When Jonah has a kicking, screaming meltdown inside her small hatchback as she drives him and his 8-year-old sister Sarah (Lia Frankland) to school, Reitman cranks up the sound until the audience feels insane and then abruptly cuts to an aerial view of her car looking small and serene. Her misery is going unheard.

Theron looked happier driving across a post-apocalyptic desert with a car of frightened slave girls. The terrifying former high school beauty queen she played in Young Adult would glance at Marlo and sneer. Craig compares her to a snuffed-out match, but the metaphor isn’t quite right. She’s drowning on land, and at night she dreams of a mermaid swimming effortlessly in the deep. Why a mermaid? It’s unclear. But it’s worth noting that they’re physically unable to spawn.

One night, Craig takes Marlo to his basement tiki bar, pops open a paper umbrella, and announces that he’s bought her a month with a millennial Mary Poppins, a night nurse who will watch the new baby so she can sleep. Marlo says no. She’s stubbornly independent. Soon after, her new daughter is born in what might be cinema’s only deliberately boring birth scene. Whenever Reitman checks in with Drew or Marlo, they’re watching TV or checking their phones, as if their miracle of life is as thrilling as a tooth-cleaning. And the film holds off on telling us the girl’s name, so that when Marlo finally mentions it, it feels like an afterthought.

There’s a percussive montage where Marlo’s sleepless nights turn into a march, the clap-clap-clap-clap beat of a light switching on, an unsnapping onesie, a slammed diaper pail, and a peck on the check, that builds until the collapse that finally convinces the exhausted mom to give the nanny a call. And when Tully (a wonderfully bizarre Mackenzie Davis) arrives this magical hired hand solves everything overnight. (Okay, maybe two nights.)

Question the rapid passage of time, and Tully replies, “That’s the downside of living on a planet with a short solar day” Is she an alien? Not exactly, but next to Marlo, a milk-leaking, primal mammal, the 26-year-old seems barely human. Her smile is almost too big, her eyes sparkle with delight. As for Tully’s flat abs, Theron channels her longing in a glance.

Tully makes cupcakes. Tully cleans. But the best thing about Tully is that she’s honest, unlike everyone else in this polite suburban purgatory where no one says what they mean. When Jonah’s principal calls him, “quirky,” Marlo snaps, “Do I have a kid or a f—king ukelele?”

Cody has a tremendous ear for dialogue. She hears the offhand mutters that speak volumes about a character, and lines that sound strange but ring exactly right, like when Marlo goes on a walk after Tully’s arrival and grins, “It’s like I can see color again.” And she loves writing for Craig’s spoiled wife Elyse (Elaine Tan), who has a two-pound dog named Prosecco.

Toward the end, the script can’t resist hitting its metaphors too hard, leading to a scene where Marlo drives an hour to pound on the door of her old loft while Tully yells that their adventure has already gone too far. Yet, despite a plot twist that falls flat, the otherwise light-fingered film leaves space for life in the margins. We have room to fill in the scenes the script doesn’t even need to show, to worry about Marlo’s neglected daughter Sarah and her own career frustrations, and take comfort in a glimpse of a teacher who could give Jonah the attention he needs. Cody and Reitman might be each other’s Tully and Marlo. Their work is 10 times better together than when they’re apart. But Tully was worth the seven year wait since the release of Young Adult. It’s a marvellous movie about the lies we tell ourselves to stay sane—and the reasons why we might need to tell the truth.