There is a surprise twist in director Jason Reitman's "Tully," about a mom whose domestic struggles increase exponentially when she adds a third child to her already challenging brood.

 

Ordinarily, this wouldn't be big news. Lots of movies toss us an O. Henry in the ninth inning, some preceded by clues, others with no justification other than that they had no other way to end the thing. Y'know, when in doubt, drop the deus ex machina on 'em. But here, everything — the plot, the message, the who, what where and when — hangs on that curveball.

 

If nothing else, the story construction alone deserves kudos. Not because it is nifty, but because it serves as an integral part of the heartrending social import achieved by the well-strategized beguilement. You think you're watching one movie and then, in a flash of revelation, the tale jumps tracks, puts all that has transpired into perspective, and teaches vital lesson.

 

"Wow, well," I thought, "OK, oh ... so that's it, huh? So all along, really, she. That's why she, hmm ... now I get it." Yeah, that kind of thing.

 

But hold off, you Sherlock Holmes types. This isn't your meat. I'm reminded of Sid, the engineer my big sister Ann dated. While probably a good catch by early 1960s standards, her chief complaint was his incessant explanation of how everything was done in the movies they watched, including, but not limited to, the technical feats and how it would all be resolved. I didn't like him because he had real shiny, dark, slicked-back hair, and vociferously stated my objection. I don't know how much my petition figured in his eventual banishment.

 

However, just in case Sid is somewhere happily ensconced in domestic bliss with a gal who appreciates a know-it-all around the house and is reading this review (Hi Sid), here's a clue: It's a chicken and egg situation, the stated premise ultimately and totally dependent on the divulgement, and vice versa. Plus, as this is not a crime drama, but closer in ethos and style to something M. Knight Shyamalan might have fashioned if he wanted to present a treatise on postpartum depression, there is no glory in divining the twist. So just be quiet if you get it, Sid.

 

Fact is, while the story is told in deceivingly lighthearted if not cynically caustic terms by a solid cast featuring Charlize Theron as Marlo, the beleaguered mom in question, this is serious stuff — even if we don't always know it. Dad, played by Ron Livingston, though paying sympathetic lip service to mom's trials and tribulations, is busy out there in the dog-eat-dog economy, the bacon he brings home his exoneration for not being fully involved. Whereas Marlo

is freaked, and already up to her ears in Herculean tasks when the new bundle of joy arrives.

 

You see, while her little girl, Sarah (Lia Frankland), 8, seems pretty normal, 5-year-old Jonah (Asher Miles Fallica) is exhibiting periodically aggressive behavior that has thus far eluded diagnosis and raised the hackles of administrators at his tony private school. So it only figures that Marlo would jump at the proposal when her rich brother, Craig, played by Mark Duplass, offers the gift of a night nanny for as long as she might need her.

 

Enters the nursery, Tully: part Mary Poppins, part Mother Theresa and part Kahlil Gibran in a 23-year-old, neo-hippie personage. Played with vivacious allure and no small amount of mystery by Mackenzie Davis, she is a dream come true. Marlo, suddenly enjoying practically full nights of sleep, can now bake cookies for the school picnic, just like the other moms, fix her hair, and maybe even enjoy a little me time. But as human nature would have it, we're worried, concerned that it's too good to last.

 

Therefore, while we vicariously luxuriate in the reprieve from drudgery Tully has made possible, we're consigned to hoping that we're probably wrong, that waiting for the other shoe to drop is merely a function of our paranoia. Nah, it's going to be just fine. But then, what would be the purpose of the film? Mere advertisement for hiring a nanny? Hence, we look for signs.

 

And in due time, as previously forewarned, we are thrown for the proverbial loop. Yet not in the way we suspected.

 

A bit drained and knowing we've just witnessed not an ad for night nannies, but what amounts to an important, feature-length public service announcement, we head to the diner for an après theater mulling. It's obvious. If we humans are all in this together, it means understanding the other gender's needs. To this aspiration, "Tully" examples an anguishing disorder that's been right under our noses since time immemorial, and to which no enlightened society should turn a blind eye.

 

"Tully," rated R, is a Focus Features release directed by Jason Reitman and stars Charlize Theron, Mackenzie Davis and Mark Duplass. Running time: 95 minutes