Based on Rachel Yoder’s 2021 novel, “Nightbitch” fits pleasingly into the film resumé of writer-director Marielle Heller by not meeting whatever expectations the trailers suggest. You watch Amy Adams turning into a dog, either for real or in her sleep-deprived parenthood imagination, and you think, well, that looks like, what? A horror film, I guess. Or a horror comedy. Maybe “Nightbitch” is the latter. But at its heart, it’s a bittersweet and calmly perceptive tale of two parents, one preschool-age child and a marriage on the verge of collapse. However vague and malleable, the terms of the couple’s parenting duties have not been set with both adults in mind. Surrounding that story is a flying leap into the fantastic, with the unnamed mother, played by Adams, going canine. She becomes ravenous for red meat. Playtime with her son becomes a joyfully intimidating dog-impersonation improv. It’s a feral means of pushback against one woman’s increasingly bleary, what-day-is-it life with a tiny, busy, demanding charge and without enough relief. For the record, her husband says, on the way out the door for another work trip, “I would love to stay here every day with him. For what it’s worth.” Adams’ nonverbal reaction to this line, delivered by Scoot McNairy as the husband with just enough cluelessness and self-deception, will strike a horribly familiar chord for anyone who’s been there, or done that. The opening in “Nightbitch” works like a charm, with mother and boy at the grocery store, again, rolling through the aisles. Adams’ character stops to chat with someone she knows. It must be so fulfilling to be a stay-at-home mom, the other woman says. Adams answers in a delirious deadpan monologue, charged by exhausted panic and dispatches from an ever-shrinking universe, though it’s all in her mind. Elsewhere, in sharply cut and paced sequences of one more breakfast with the boy, director Heller and editor Anne McCabe chart these daily routines with witty, accelerated precision. As the dog transformation continues, the mother grows a tail and some fur in some unusual places and starts running with a mysterious pack of neighborhood dogs at night. Curious, to say the least, about this development, she consults a local librarian (Jessica Harper, excellent as always) for any books on hand to explain her transformation. There is one, it turns out, a field guide to magical women. Harper’s character senses what’s happening with this isolated, increasingly desperate woman stuck in “a 1950s marriage.” When the mother starts telling more of the truth out loud, instead of just muscling through the resentment, the unwinable difficulty of it all finds the oxygen she needs to make things better. Amy Adams and Scoot McNairy in “Nightbitch.” (Anne Marie Fox/Searchlight Pictures) I love the extended confrontation climax, which has the push, pull and authentic pain of real life, but the happy ending feels pushed and preordained. McNairy’s performance is pretty good, but even in a relatively restrained characterization, McNairy (very good as Woody Guthrie, by the way, in the forthcoming Bob Dylan movie “A Complete Unknown”) seems shifty and untrustworthy, and that makes the Adams character seem forgiving to a fault. The movie’s tonal change-ups vary wildly, mostly for the good. In every Heller movie to date, from “The Diary of a Teenage Girl” to “Can You Ever Forgive Me?” to “A Beautiful Day in the Neighborhood” to this one, the tonal mixture is challenging. It’ll vex some viewers accustomed to more hand-holding. Adams, though, works like magic glue to keep it all together. At its best, “Nightbitch” is many things at once: funny, unruly, bizarre, tender. I reject the idea that “Nightbitch” will appeal mainly to parents (especially mothers) in something akin to the tipping-point scenario depicted by Yoder, and by adapter Heller. Working toward a humane work/life balance? Easier said than done. But it needs doing, if finding “a more equitable way to do this parenting thing,” as we hear spoken aloud in this clever fable, hasn’t come a little too late. “Nightbitch” — 3 stars (out of 4) MPA rating: R (for language and some sexuality) Running time: 1:38 How to watch: Premieres in theaters Dec. 6 Michael Phillips is a Tribune critic.