When an actress gives a four-star performance in a two-star movie, what is the reviewer to do?

Splitting the difference seems to be best option.

“Still Alice” is at times maddeningly overwrought and heavy-handed in the telling of a 50-year-old woman (Julianne Moore) who has an idyllic life until she is diagnosed with early-onset Alzheimer’s.  One of the more intrusive scores in recent memory announces every scene in which Alice will get lost in her own house or forget the name of one of her children. A plot development rich with dramatic opportunity is utterly neglected. And there are times when the dialogue seems straight out of a low-budget Lifetime movie.

What makes “Still Alice” worth the journey is Julianne Moore’s brilliant and delicately calibrated performance as an Ivy League linguistics professor, devoted wife and mother and almost annoyingly accomplished woman who gradually descends into an almost totally helpless, deeply frustrated and tragically lost human being who knows she is going to lose herself and is ultimately powerless to do anything about it.

Directors Richard Glatzer and Wash Westmoreland, adapting Lisa Genova’s novel, introduce us to Alice’s world while things are still about as perfect as a life can be. Alice is a widely respected teacher at Columbia and the author of the book “From Neurons to Nouns.” Her husband John (Alec Baldwin, the go-to guy for roles such as this) is a successful senior research physician. They have three great-looking, adoring children: Tom (Hunter Parrish) a medical student; Anna (Kate Bosworth), who’s trying to start a family, and Lydia (Kristen Stewart), who wants to be an actress.

A house on the Upper West Side where every meal looks like it’s been prepared by chefs on a competition cooking show, another home on the beach, the family, the career — Alice has it all.

Until she gets lost while jogging on campus, and she can’t remember the words for familiar objects, and she visits a neurologist (Stephen Kunken, in a wonderful performance) who administers some tests and has to deliver the terrible news: Alice has early-onset Alzheimer’s, a rare and familial strain of the disease, mostly likely inherited from her father.

Which means each of her three children is at risk as well.

The scene in which Alice delivers the news of her condition to her children and has to tell them they might have inherited the disease is heartbreakingly effective. Two of them opt for tests that will tell them definitively if they’ll have early-onset Alzheimer’s; one would rather not know.

One can imagine the conversations each of the offspring must have had before deciding what to do, and how one would react upon getting the verdict. One has to imagine this, because the filmmakers opt to dismiss this subplot with a throwaway phone conversation, and then it never comes up again.

Alec Baldwin’s John loves his wife and does his best to support her, but the screenplay paints him as a selfish workaholic who seems less and less invested in Alice’s plight and more concerned with a big job opportunity. Of the three grown children, Kristen Stewart’s Lydia is the most interesting and complex character, and Stewart does excellent work. Lydia is the closest thing to a black sheep this family has, but she’s clearly the closest to Alice as well. That she doesn’t tiptoe around her mother’s condition draws them even closer.

For the umpteenth time, a film uses “old home movie footage” to yank at our heartstrings. (Such footage almost always includes “That Perfect Day at the Beach” scenes.) Maybe there’s no better way to make the point, but it would be nice to see someone try.

Of course a film with subject matter such as this isn’t always easy to watch. We know we’re going to get scenes where Alice confuses the names of her children, explodes in the middle of the night because she can’t find something, stops worrying about her appearance.

Moore has been the odds-on favorite to win the Academy Award for her performance, and with good reason. First, it’s the kind of showy part that wins awards; more important, Moore delivers a performance that should win awards. We believe every inch of the performance, every movement of Moore’s eyes when she gets the news of her condition, every scene in which she experiences another level of deterioration.

It’s beautiful work.

[s3r star=3/4]

Sony Pictures Classics presents a film written and directed by Richard Glatzer and Wash Westmoreland, based on the novel by Lisa Genova. Running time: 99 minutes. Rated PG-13 (for mature thematic material and brief language including a sexual reference). Opens Friday at Landmark Century Centre.