Five years after Kate and Geoff Mercer were forced to cancel their 40th wedding anniversary due to Geoff's heart bypass surgery, the affluent, childless couple from Norfolk are gearing up to celebrate their 45th anniversary surrounded by friends at the Assembly House in Norwich. However, their plans take an unexpected turn when Geoff receives a letter revealing that the body of Katya, his former German lover from the early 1960s, has been uncovered in a melting glacier—where she tragically fell into a crevasse during a hike in Switzerland over fifty years ago.
Although Kate has previously learned about Katya from Geoff, she initially appears unfazed by his growing unrest. Geoff explains that he and Katya had posed as a married couple to share a room during the more conservative 1960s, leading the Swiss authorities to regard him as Katya's next of kin.
As the days progress and the anniversary celebrations approach, Geoff's mood darkens, prompting him to revert to smoking. One night, while rummaging through the attic, he looks through memorabilia associated with Katya and hesitantly shares a picture of her with an increasingly agitated Kate, who is struck by the resemblance between herself and Katya when they were younger, both sporting dark hair.
While Geoff is at a reunion luncheon, Kate decides to venture into the attic, where she discovers his scrapbook filled with mementos from his time with Katya, including pressed violets from their last hike together. In addition, she finds a carousel slide projector next to a makeshift screen that showcases their memories in Switzerland. To her horror, Kate discovers slides indicating that Katya was pregnant at the time of her death.
Feeling betrayed, Kate begins smoking again and confronts Geoff about his peculiar behavior after discovering his interest in trips to Switzerland—without disclosing her attic findings. She expresses her belief that many of their life choices have been unwittingly influenced by Katya. Geoff reassures her that their marriage will undergo a "fresh start," which he attempts to demonstrate the following morning by serving breakfast in bed.
The anniversary party takes place in the historic Grand Hall, where Geoff delivers an emotional speech, affirming his love for Kate. As they prepare for their first dance, the same song that played at their wedding—"Smoke Gets in Your Eyes" by The Platters—fills the air. While Geoff exuberantly dances, Kate starts to feel increasingly uncomfortable and stiff. As the song concludes, Geoff raises their hands in celebration, but Kate abruptly pulls her arm down, leaving her feeling isolated amidst the revelery. Despite the joyous occasion, she finds herself alone on the dance floor, contemplating the rift forming between them.
