“Still Alice” is an incredibly sad coming-of-age story. Most mainstream entries feature an adolescent making the mental leap from child to adult. This story follows the life of its protagonist from middle age to mental childhood.

If it didn’t star Julianne Moore, it would be well worth skipping. But the exceptional way Moore plays Alice, a renowned Columbia University linguistics professor with early-onset Alzheimer’s, lifts the film above its morose potential. You don’t need to have a personal connection with the disease for the film to abscond with your heart. Just watch Moore’s work.

It is not a perfect film. It’s narrowly focused, like a cable movie about the plight of a decent, upstanding upper-class woman, the kind of slim blues riff Lifetime runs three a week. Directors Richard Glatzer and Wash Westmoreland made earlier films where they couldn’t grasp the material firmly, a bland Kevin Kline yarn about the late movie star Lothario Errol Flynn (“The Last of Robin Hood”) and the cheesy gay porn satire “The Fluffer.” Adapting this film from neuroscientist Lisa Genova’s bestselling novel, they are authentic. You admire their push for playing the facts of a critical illness straight and carefully, making sure that every point is accurate and that every expressive close-up is held as long as possible.

Alice is 50, married to a doctor (Alec Baldwin) with his own important academic professorship. The brilliant couple live in a beautiful Upper West Side townhouse, and love meeting with their three grown children: Anna, who is pregnant with twins (Kate Bosworth); Tom, a young doctor (Hunter Parrish), and Lydia (Kristen Stewart), the rebel of the pack, who is trying to pursue an acting career. The texture of the early domestic scenes is nice, like Alice’s interactions with her students.

Her specialty is research into the way small children learn semantics, and when her grasp of language begins to slip away, she takes tests privately, then tells all four of her tribe what’s coming. From Baldwin on down, they are stunned.

The movie isn’t about marriage. Its entirely believable focus is how the whole family responds to the fragmenting, disintegrating illness that has invaded Alice. We see the squabbling it creates, the feelings that overlap or collide and the laughs when Alice is soothing and adjusting on a good day, trying to keep tension to a minimum. The reason the film flies is the directors’ long, touching shots of Moore. One truly saddening sequence has her trying to prime herself to deliver a lecture, only to discover that her major asset isn’t on its best behavior that day.

As Alzheimer’s snips her cranial nerves, the insoluble mystery of her sickness grows while her attention shrinks. She remains a good and tender woman as the world becomes populated by strangers. When Anna delivers her twins, her mother’s wish to embrace them in the hospital worries the family. Could she do that safely? “I know how to hold a baby,” she says, simultaneously hurt and proud.

The title is a key to the film’s meaning. Is “Still Alice” a way of saying that, for all her struggles, Alice is still committed to staying among us? Or does it imply the stillness that arrives at the end of life? As her perceptions shrink, Alice withdraws deeper into youth. Moore plays her at a dozen different age spans over the course of the film. In a stunning sequence, forgetful Alice discovers a video she recorded for herself a year earlier, offering important secret advice. It’s as if Moore is playing two utterly different women having a Skype conversation.

She can carry the film through ordinary scenes — having a tough phone conversation while on a walk, chopping vegetables for dinner with a large knife, angrily wondering where an important item got put away — and make them electrically powerful.

The reason Oscar conversations tend to put Moore as a best actress front-runner is not that she’s “due,” as if being in the race repeatedly before makes you a winner now. It’s because she is an artist of limitless resources, even when she’s playing a woman whose human properties are draining away.