Fearless Charlize Theron is a chameleon. Donning a fat suit and unflattering make-up, she won an Oscar as the serial killer in “Monster,” she was totally convincing as the warrior Furiosa in “Mad Max: Fury Road,” and she kicked serious butt in “Atomic Blonde.”

Now, she’s piled on nearly 50 pounds to play Marlo, a bleary, seriously stressed-out, suburban New Jersey mother who, much to the amazement of her uncomprehending husband, Drew (Ron Livingston), hesitantly accepts her wealthy brother Craig’s (Mark Duplass) gift of a “night nanny” after she gives birth to her third child.

That’s a home health-care aide who comes in after dark and stays all night, quietly bringing the newborn into the bedroom for breast-feeding, allowing its mother to get some much-needed sleep.

“They’re like ninjas,” Craig explains. “They sneak in and out. You barely know they’re there.”

But when svelte, 26 year-old Tully (Mackenzie Davis), appears on their doorstep, Marlo - having seen far too many “The Hand That Rocks the Cradle”-inspired thrillers - is, at first, reluctant, to turn over care of baby Mia. But she’s completely exhausted, overworked and overwhelmed.

Gradually, Marlo genuinely warms up to earthy, eager-to-please Tully, who insists on caring not only for Mia but also for Marlo, who has her hands full with challenging, eight-year-old Sarah (Lia Frankland) and ‘quirky’ kindergartner, Jonah (Asher Miles Fallica), whose uncontrollable behavior places him somewhere on the autism spectrum.

You can witness definite progress being made until, one fateful night, the empathy that ostensibly connected the two women is sorely tested. To say more would ruin the plot’s unexpected postpartum twists and turns.

Adroitly written by Diablo Cody and uncompromisingly directed by Jason Reitman, who collaborated previously with Charlize Theron on “Young Adult,” it wobbles on a very thin line of credibility…and it’s certainly not your typical Mother’s Day movie.

Nevertheless, on the Granger Movie Gauge of 1 to 10, “Tully” totals a 7, even if - when you think about it later - it doesn’t add up.