In a culture obsessed with youth, we’re constantly informed that sex is the glue that holds a good marriage together. But as the years pass and desire fades, most happy couples will tell you the stronger bond is shared memory: it’s the history spouses draw on every day, informing all their in-jokes and knowing glances and loving accommodations. Without memory there’s no real understanding, because the past is where all lessons dwell, and there’s no real intimacy, because the present belongs to everyone. Old friends are the best friends not because they’re old but because they remember us young.

Away From Her, a powerful Canadian drama adapted from Alice Munro’s 2001 story “The Bear Came Over the Mountain,” asks whether any love can endure once that shared history is wiped away by Alzheimer’s disease. The two central characters, Grant (Gordon Pinsent) and Fiona (Julie Christie), have been married for 44 years and enjoy a fond and intimate relationship, living out their retirement in a comfortably rustic house in small-town Ontario. But Fiona’s memory has grown so spotty that, as she puts it, she’s “beginning to disappear.” After she wanders off one day, forcing Grant to rescue her, the couple must confront the reality that he can no longer care for her properly. Fiona decides to check into Meadowlake, an assisted living facility, and as her memory of Grant dissolves, the only place their marriage survives is in his mind.

The movie is the feature writing and directing debut of accomplished Canadian actress Sarah Polley (The Sweet Hereafter, Don’t Come Knocking, Dawn of the Dead), who, at only 28, proves remarkably attuned to the texture of a relationship that’s weathered decades. Grant and Fiona have been together so long they barely need to speak; their telegraphic conversations are seasoned with little conspiratorial jokes, and observations can be rendered with the slight raise of a chin. Their playfulness springs from a deep well of hard-won wisdom. Reading aloud from a book on Alzheimer’s, Fiona tells Grant that a caregiver must be cheerful in the face of abuse and patient when the other is upset for no apparent reason. “Sounds like a regular marriage,” she adds. At another point Fiona pretends to have forgotten something, worrying Grant, then chirps, “Just kidding!” Relieved, Grant responds with a lighthearted “Fuck off!”

But the memories uniting the two aren’t always pleasant. Grant is a retired English professor, and when they’re driving to Meadowlake Fiona makes oblique reference to his affairs with his students. “All those bare toes, all those sandals,” she recalls, conjuring up a vivid image of temptation in the classroom. Grant is silent, consumed with guilt, but Fiona is gracious: instead of torturing him with recriminations, she thanks him for having stuck around when all his colleagues were ditching their wives for younger women. “People are too demanding,” she says. “People want to be in love every day.” The scene is beautifully underplayed, a casual moment fraught with conflicting emotions. Fiona may have forgiven Grant for his philandering, but she sure as hell hasn’t forgotten.

Unfortunately, Meadowlake seems almost purposely designed to erase the past. As the nursing home’s starchy director explains to Grant, no visitors are allowed for the first 30 days; the policy is supposed to help newcomers “settle in,” but as one nurse points out, it benefits the staff more than the residents. A big-screen TV provides constant present-tense distraction, and the bland contemporary music seems to have been chosen by the young staffers. Eventually Grant realizes that the laundry service carelessly redistributes the residents’ clothing, so they wind up wearing one another’s things. Most of them are too far gone to notice, but he’s appalled when he arrives one day to find Fiona wearing a tacky sweater she never would have tolerated at home. For the sake of others’ convenience, her personality is being pried away from her.

The couple’s initial separation is heartrending: out in the parking lot Grant begs Fiona not to go, but she gently admonishes him, puts on her bravest face, and announces herself at the front desk as if she were merely checking into a hotel. Shown to her room, she asks the director for some privacy and makes love to Grant. When they’ve finished, she implores him to leave: “If you make it hard,” she says brightly, “I might cry so hard I’ll never stop.” Back at home, alone for the first time since they were married, Grant stares at himself in a mirror, and its reflection shows him as a young man, Fiona appearing from behind to slip her arms around him.

As the public’s understanding of Alzheimer’s has increased, the disease has made its way into movies that range from tragedy (Richard Eyre’s British drama Iris, about the slow deterioration of fiction writer Iris Murdoch) to schmaltz (Nick Cassavetes’s three-hanky hit The Notebook). But Alice Munro is a tough-minded storyteller, and the twist that occurs midway through Away From Her precludes any sentiment. When Grant returns to Meadowlake after the 30-day separation, not only has Fiona forgotten him entirely, she’s attached herself to another resident, a mutely glaring old man named Aubrey (Michael Murphy). The nurses have clearly coached her to expect a visitor, and while she welcomes Grant, she does so with the formal politeness of a stranger. Grant can only look on in shock as Fiona and Aubrey sit in the TV room together, watching a golf match, her responses matching his as if they were the old married couple.

This nightmare continues with each successive visit, and eventually Grant begins to wonder if Fiona might be exacting her revenge for his past infidelities; even a scenario as cruel as that seems preferable to the idea that 44 years of marriage can evaporate in a month. Once Fiona has forgotten their history, it becomes a heavy burden to Grant. The horror never seems to end: as Fiona pushes Aubrey down the hall in his wheelchair, a sheet of paper sails off his tray, and Grant, following behind them, scoops it up to discover a lovely pencil sketch of Fiona that restores her to her youthful beauty. Later, when Aubrey’s harshly disappointed wife (Olympia Dukakis) decides to care for her husband at home, Grant walks in on Fiona comforting the distraught old man, stroking his hair and calling him “dear one.”

For a movie about the importance of memory, Away From Her is appropriately sophisticated in its treatment of time. Polley has broken the chronological story into three sections of unequal length and woven them together, approximating our own mercurial journeys through the past. (Her own husband, David Wharnsby, served as editor on the film.) The longest section finds Grant driving out to Aubrey’s house to ask the wife to take him back to Fiona. For Grant it’s the supreme sacrifice, yet that spirit of generosity is what enables a marriage to flower for decades, and the wise spouse eventually learns to see sacrifice as an investment rather than a loss. In this case the gift seems all the more poignant because Fiona may never recognize it, much less remember it. But Grant will, and he’s the only one who has to live with the past. His decision adds a necessary note of hope to an otherwise agonizing tale: you can never rewrite your history, but you can always amend it.