Movies built around mental maladies have a couple of consistent shortcomings, both of which the Teresa Palmer OCD dramedy “Addition” struggles with, start to finish.

There’s a tendency to treat something serious as cute and quirky. Obsessively counting everything from servings in a dim sum dish or eyelashes or bananas in your bunch in the checkout line kind of fits. Seeing visions of obsessive counter and electronic genius Nikola Tesla, who eggs our heroine into counting the bristles on her toothbrush — ALL 90 of her toothbrushes — also ticks that box.

And then there’s the suggestion that whatever psychotherapy and drugs offer, only “love” can save our disordered hero or heroine at day’s end. That proves to be the hardest trope to let go of in Marcelle Lunam’s Aussie film, based on a novel by Toni Jordan.

Becca Johnstone’s script has us spend most of the movie inside heroine Grace’s head as she voice-over narrates her every thought, taking us inside the processes that movie screenplays suggest bedevil people with OCD.

“No, obession is not a weakness,” Grace rationalizes. “Life without obsessing over things is no fun at all.”

She counts when she wakes up. She counts when she walk and counts a little when she teaches her niece’s (Lou Baxter) high school math team. She counts the tables at her only restaurant, counts the bites in her orange cake via the poppyseeds she counts in the frosting, and the waitress there has an “As Good as it Gets” tolerance of her weirdest customer.

She counts when she goes to sleep and when she wakes up, with the mustachiod Serb Tesla (Eaman Farren, given nothing to play or say), who eggs her on from bed, purring “Iljabuvi,” Croatian Serb for “my love,” after every encouragement.

It’s the counting that counts when lovely thirtysomething Grace has her “meet cute.” She swipes a banana from a distracted customer (Joe Dempsie) in line with her at a market. The “distracting” is easy because “I’m good at flirting.” Seamus, as the Brit-born house-painter is named, is no slacker at flirting himself, even if he’s a little slow about the obvious.

“You’re a bit odd, aren’t you?”

The path to romance takes us through family history, the class divide — she’s a Phd and a “genius,” he’s a mere “tradie” (tradesman) — rising expectations doomed to be demolished and of course, another sort of flirtation, with a trained psychotherapist whose novel approach is a sort of “Go or stay” attitude about a patient who obviously needs her.

The lead performances have a tenderness that makes this work better than you’d think, considering the treatment of the subject matter isn’t much more serious than your average “Big Bang Theory” episode.

Palmer’s outgrown her “Aussie Kristen Stewart” labeling, even if she never flowered into the mainstream success that seemed her destiny.

Our star and her director Lunam — the documentary “Habana Shakes” was hers — are limited by the confines of the script, which circumscribes the dramatic possibilities and makes outlying situations and characters, past and present — the teen niece, her hovering, over-achieving mother (Adrienne Pickering), Grace’s older sister, and their mother (Sarah Peirse) — feel as shortchanged as poor Tesla.

Which is a final verdict on “Addition,” that it never quite adds up to what it might have been.

Rating: TV-MA, sex

Cast: Teresa Palmer, Joe Dempsie, Lou Baxter and Eaman Farren.

Credits:Directed by Marcelle Lunam, scripted by Becca Johnstone, based on a novel by Toni Jordan. A VRC release on Amazon Prime.

Running time: 1:31