How well do you know your partner? How well do you think you would—or could—know them, if given forty-five years together? Andrew Haigh’s 45 Years is a film about marriage but concerned with knowing, and it is a devastating, slow-motion vivisection of a marriage shaken by knowledge. Charlotte Rampling and Tom Courtenay star as Kate and Geoff, a long-married couple about to celebrate their forty-fifth wedding anniversary. The film unfolds over the course of the week leading up to their big anniversary party, and what at first seems like a bulletproof marriage is revealed to be something else entirely.

Adapted and directed by Haigh, 45 Years is a stripped-down drama, largely made up of spare, static takes and long, dialogue-heavy scenes that mostly occur in domestic spaces. Haigh wastes no time setting up his characters—within moments of the film beginning we know that Kate and Geoff have long since attained the comfort of long-term couples, moving easily around each other and picking up conversations they left off sometime before. Kate, particularly, seems happy, a retired school teacher content with her life in rural-ish England, accompanied by her husband and dog.

But Haigh quickly introduces conflict in the form of a letter. Just five days before their big anniversary, Geoff receives a letter stating that the body of a previous love has been found in the Swiss Alps. Her name was Katya and she died while hiking through the mountains with Geoff fifty years before. Kate knew OF Katya, but over the next week, she learns ABOUT Katya, and what she learns irrevocably alters her relationship, threatening not just the time she and Geoff have left, but also rewriting their whole history together.

Courtenay gives a strong performance as Geoff, portraying him as a man who has already been physically diminished and is now slipping into the past in a way that leaves him more than usually doddering. But this is Rampling’s film from beginning to end, and she holds the screen effortlessly. Kate’s peppy walk at the beginning of the film gives way to a trudge, and her shoulders slump more each day, but the real horror is in her eyes, which slowly fill with terrible knowledge. Rampling is utterly devastating doing nothing more than looking at slide photographs, and the final scene, in which Kate struggles amidst the sincere cheer of her anniversary party, is crushing. Seriously, the ending of 45 Years is a “sit quietly in the dark and regroup” moment.

From a certain angle, 45 Years is a ghost story, with Katya’s specter just out of frame in every shot. But that also oversimplifies Katya’s role in Kate and Geoff’s marriage. It’s too simple to say that she haunts them and it’s missing the point to assume Kate is some kind of replacement. It’s more like Katya represents a timeline diversion, and Kate, who thought she was living in the happy timeline, finds out she’s actually in the darkest timeline. It isn’t that Geoff withheld specifics about Katya or that he undersold their relationship to Kate whenever it was that he first told her about it, it’s that as Kate learns more about Katya, she begins to see how her entire life with Geoff is shaped by choices she had no idea he was even making, all reactions to Katya.

Even as the space between them grows, Haigh finds ways to show us the deeply loving, committed couple Kate and Geoff are/were. There is no question that Geoff sincerely loves Kate, but that just makes it all the more painful as Kate understands the role Katya really played in his life, and how that carried into their marriage. 45 Years is harrowing, offering no pat resolution or easy answers. There are only questions, and whether or not a person can ever really withstand knowing the answers.