One of the more awkward topics in the early points of a romantic relationship involves the discussion of past loves. The reality is most people are not their current love’s first love, and yet some struggle to admit there was someone before them. This topic can be most sensitive in the early months of a relationship, especially if there is a concern that feelings for an ex might still exist. Fear about this isn’t exclusive to new relationships, however. In Andrew Haigh’s sublime 45 Years, a couple who has been together for nearly half a century finds their relationship suddenly tested by a voice from the past.

That couple is the Mercers: Kate (Charlotte Rampling), a retired teacher, and Geoff (Tom Courtenay), a retired plant worker. They live a quiet life in the British countryside where they go about their business the ways most retired couples do: walking the dog, puttering about the house, running errands in town, etc. Those halcyon days of their golden years take a sharp turn just a week before their 45th wedding anniversary, when Geoff receives a letter that the body of a long-deceased former love has been found. “My Katya,” as Geoff refers to her when he breaks the news to his wife, was the love he knew before Kate. The discovery of Katya, whose body was frozen solid and lost for half a century in the mountains of Switzerland, changes Geoff. That change, along with the subsequent discovery of other information, changes Kate.

There’s a high degree of difficulty in properly presenting 45 Years without it devolving into some mawkish soap about old age and young love and regret and whatnot. Fortunately, it’s a challenge Andrew Haigh (who adapted the screenplay from David Constantine‘s short story In Another Country) more than rises to. The filmmaker has a keen awareness that a 45-year marriage is simultaneously strong and vulnerable, and he has a clear understanding that the frailty of the human ego is something that doesn’t fade with age.

The strength of the Mercers’ relationship is the most obvious aspect of the film. A couple doesn’t get to its 45th wedding anniversary on cruise control. Marriages take work to get that far, and the Mercers have put in that work, but their success is measured by more than just a number. It’s also measured by their contentment and ease with each other. It’s a subtle but important thing. This is an elderly couple not portrayed as bitter or cantankerous or even slyly dismissive of each other; they love each other and have for a long time. The fact that they are planning a 45th-anniversary party is a great example of that. They had planned a party for their 40th—a more logical milestone—but illness got in the way. They didn’t reschedule it for as soon as possible, nor did they clamor to try again at 41. They shrugged their shoulders, knew in their hearts they’d be together no matter what the year, and rounded to the next 5-year marker to throw a replacement party.

The part that’s less obvious, the part that’s more important, is the vulnerability of a relationship that has lasted so long. It isn’t a vulnerability that comes with boredom or complacency because these aren’t people looking for something new. In fact, it’s quite the opposite. These are people who are comfortable with, perhaps subconsciously cling to, the familiarities and rituals they have built up over 45 years. The film is rich with little suggestions of this. So when something like the unexpected reemergence of the corpse of a past love enters this familiar space, it might not crumble the house, but it chips the paint, and chipped paint is the kind of thing that gnaws at someone because they know it’s there and they can’t leave it alone.

And therein lies the never-aging frailty of the human ego that Haigh gets so right. With the reemergence of Katya’s body, Geoff is whisked back to the past and once again reminded of a love realized and yet left incomplete by tragedy. If he were a 40-something who had run into a high school flame at a reunion, he might buy a flashy car. He’s not that guy. Instead, he starts smoking again. He tries reading Kierkegaard again. He moves a little closer to being that irritable old man who wonders if he did it right. He worries that his old love’s frozen body has not aged a day while his has aged thousands. These little changes, these little comments, this renewed interest in a time he long filed away keeps the paint chipping and threatens to crack a wall.

Kate is in tune with it all. Acutely.

At first, it’s not that big a deal. Sure, it’s an old love, but it’s a dead love. However, as Geoff’s interest in Kierkegaard and finding old mementos increases, and as those moments when the couple would share quiet small talk turn into a discussion about Katya (again), Kate wears down. She asks questions—little ones—that illustrate the stoic and supportive face she wears on the outside hides an unraveling self-confidence on the inside. Learning something new and unexpected only exacerbates the problem because now it feels like Geoff is hiding something. When she starts poking around in the attic, her disbelief is crippling. The stakes are immeasurable because it’s not as if she might lose her husband to some fling the way a 40-year-old might; she might lose her husband to a ghost, and there’s no fighting that.

Rampling plays her incredibly deep and complex role to perfection. There is no scenery to chew, no impassioned speech to make, no confrontation to be had with “the other woman,” so in the absence of that, Rampling wields subtlety like a surgeon with a scalpel: precise, efficient, effective. It’s an amazing performance, and one made greater by the fact that Haigh keeps her the focus of almost every scene. But Courtenay is no slouch either, and it takes a real actor to be convincing in his late-life change and give Rampling everything she needs to shine.

Love does not have a finish line. There is no point along the timeline of a relationship where someone can say, “We made it this far; nothing can come between us now.” A relationship is like any other living thing: it needs constant care and attention, and it is always susceptible to damage, whether it’s a budding flower of romance or a mighty oak of marriage. With 45 Years, Andrew Haigh and his pair of stars prove this to be true, and they do so in the most well-measured yet mesmerizing of ways.