The great Joanna Lumley adds another blowsy, foul-mouthed grump to the often hilarious third act of her career as she steals the operatic comedy “Falling for Figaro.”

But thanks to the atonal script and flat performances around her, that never amounts to more than petty theft.

A thin comedy about following one’s dreams, no matter the odds, and reaching for the high notes, it only occasionally hits the right notes. It’s not that the singing and would-be romance in it is too sharp. The filmmakers rub all the edges off, lowering the stakes and rendering the whole affair a lot more drab than its colorful setting.

Aussie actress Danielle Macdonald (“Patti Cake$”) plays Millie, an American fund manager making hay in a London firm and living with the bloke (Shaza Latif) who had the good sense to hire her. But she’s bored enough by the work and the money to turn down a plum promotion.

“I’ve always wanted to be an opera singer,” she abruptly declares.

“You mean, like in the shower?”

As this isn’t “just a whim,” she’s advised to seek out a former diva-turned-vocal coach, Meghan Geoffrey-Bishop (Lumley of “Absolutely Fabulous”) in the hinterlands of tiny Drumbuchan, Scotland, a one-pub/inn village where the chef, handyman and bellman is the “retired” Geoffrey-Bishop’s only other student.

Max (Hugh Skinner) has dreamed Millie’s dream a lot longer than her. He’s been taught, berated and coached by Geoffrey-Bishop for years, and still hasn’t quite “got it.”

Both of the aspiring singers hope to launch their careers via a national “Singer of Renown” new talent competition.

A few blasts of profanity, a “complete amateur” dig about her “just above karaoke standard” voice, and the lessons and “competing” begin, with hapless Max falling for the woman who figures on stealing his dream.

“Falling for Figaro” goes off the rails, almost right from the start. Millie’s beau, Charlie, may doze off at the operas she insists on attending. But he’s otherwise supportive. There’s no edge to the character and little conflict or sense that she needs to come to her senses about her dream or about him.

The players don’t do their own singing, which is understandable. They don’t have any chemistry, which isn’t.

The plot has Millie wowing the “Singer of Renown” judges and “going viral” with a Mozarted-up rendition of “Twinkle, Twinkle Little Star,” which is the film’s cutest touch.

But the idea that the scheming teacher decides that her two pupils need to break each others’ hearts a little to make them more empathetic performers and raise the stakes is meekly set-up and handled. No mention is made of why this should work, not even noting how dramatic and tragic the real life of a Maria Callas was, informing her art.

The singing is never dramatic or thrilling, and the “competition” is drably presented.

Macdonald first gained notice with a character and a film that were “out there,” “Patti Cake$.” She’s to be commended for elbowing her way out of her zaftig, brazen and funny niche (“Dumplin’,” “Poker Face”). But there’s nothing to Millie. All Macdonald’s interesting edges are rubbed off like pretty much all the other characters in the movie.

“Falling for Figaro” wastes some beautifully soggy Scottish locations and pointed character turns by Lumley and veteran actor Gary Lewis as the proprietor of The Filthy Pig Inn and Pub by simply never amounting to anything anybody would want to invest in.

Rating: unrated, adult situations, profanity

Cast: Danielle Macdonald, Hugh Skinner, Shaza Latif, Gary Lewis and Joanna Lumley.

Credits: Directed by Ben Lewin, scripted by Ben Lewin and Allen Palmer. An Umbrella, IFC release on Netflix

Running time: 1:44