Jason Reitman is a filmmaker who has built his reputation around creating emotionally powerful dramedies involving unpredictable characters in such films as “Juno,” “Up in the Air” and the vastly underrated and nearly unseen “Men, Women & Children.” In two of his best films— the Oscar-winning smash “Juno” and 2012’s underrated “Young Adult”— he teamed with writer Diablo Cody, who added biting, uniquely funny dialogue to the mix with stellar results.

Now, the two are back together after six years apart with another winner, “Tully,” starring Charlize Theron (who starred in “Young Adult”) as a married mother freed from sleep-deprived insanity by a night nanny whose free-spirited approach to handling her newborn child lights up her entire family’s existence. It’s almost shocking that the film is released now, at the start of the summer season, because of the very fact that its thoughtful and humane story is a complete antidote to a month of turbocharged blockbusters. If you’re sick of superhero movies, run directly to any theater showing this film.

“Tully” opens with Theron’s Marlo, a 41-year-old mom whose looks and dreams are slowly being snuffed out by the frustrations of mothering two kids and a pregnancy, as she rushes to a meeting with her son Jonah’s kindergarten principal. Jonah is officially labeled “atypical,” a kid with severe behavioral issues who can’t handle myriad aspects of daily life and socialization with his peers. Marlo is told with wary politeness to find a one-on-one aide to handle his issues.

When she gives birth, her tenuous handle on daily existence gets put to the test in a terrific montage that shows the endless cycle of diapers, breastfeeding and interrupted sleep that any new mother endures. Her wealthy brother Craig (Mark Duplass) offers to get her a night nanny named Tully (Mckenzie Davis), whose hippy-ish charms instantly make things better even as she forms an awkwardly close bond that includes intently watching Marlo breastfeed and pump milk from start to finish.

The plot seems to just follow their touching and funny friendship and the transformative power of having just one person care enough to make your difficult life easier. But then a rowdy night on the town for the two friends in Brooklyn results in a truly stunning twist that changes everything about what we have come to understand and expect about the storyline. Thank God, it’s a twist that is totally brilliant and makes the film even more affecting.

Theron is a real pleasure to watch throughout the film, deftly alternating between her darkly comic jabs and intensely emotional sad moments while expertly conveying the exhaustion that mothers endure. It’s refreshing to see this immensely talented actress take on a humane and beautifully nuanced role after she used her statuesque looks and magnetic presence on ultra-violent fare such as “Atomic Blonde” and “Mad Max: Fury Road” in the last couple of years.

As her husband, Ron Livingston astutely portrays the kind of husband that is all too common: a decent guy who doesn’t mean to be a bad partner but travels so much for work that it’s easier to just tune out on the subtle grind of everyday home life. As he learns to step up, he shines brightly with the Everyman charm he displayed way back in “Office Space,” a cult classic that should have made him a major star.

But it’s Davis as Tully who is the true discovery here, a fresh face with a magical spirit who sometimes seems too good to be true. It’s easier to imagine her as Diablo Cody’s magnificent teen heroine Juno a decade later, a kind heart in an often uncaring world whose simple decency toward another human being changes everything.

“Tully” is the kind of movie that reminds us that any of us can be that person for someone. Hopefully, it will withstand the bombast of summer to get some loving attention of its own come Oscar season.