There are two timelines that weave through the film, interconnected by a mysterious mailbox at a seaside house called Il Mare. The story begins when Eun-joo, Jun Ji-hyun, moves out of the house and leaves a Christmas card in the mailbox, asking the next resident to forward her mail. Sung-hyun, a young architectural student and the first resident at Il Mare, receives the card, which is dated two years into the future. After exchanging messages across time, they discover they are living two years apart: Eun-joo in 1999 and Sung-hyun in 1997. The mailbox proves to be a conduit for their communication, capable of passing objects and even living beings between the years. Eun-joo eventually asks Sung-hyun to retrieve a tape player she lost two years earlier, and he does, bridging their separate existences through this uncanny channel.
As their correspondence deepens, an important connection forms between Eun-joo and Sung-hyun. When Sung-hyun’s estranged father, Kim Mu-saeng as Professor Han, a noted architect, falls ill, Eun-joo is asked to obtain a book about him, which she manages to secure, though she is hospitalized for a minor accident and cannot deliver the book in time before the elder man's death. Reading the book helps Sung-hyun finally feel his father’s love and he tentatively returns to architectural work, sensing a renewed purpose in his craft. The two lovers keep building on their bond across time, planning a date where each will guide the other through a shared experience in their own era. Eun-joo leads Sung-hyun through an amusement-park outing and, in another moment, a restaurant scene where she drinks a wine he left behind two years earlier. Even as they enjoy these solitary dates, they dream of meeting in person at a beach two years in Sung-hyun’s future.
Eventually Eun-joo travels to the beach and hopes to meet him, but Sung-hyun does not appear. He is perplexed, having trouble understanding why such a moment would slip from his memory. He is drawn to the beach’s beauty and to the idea of designing a house there for her, a dream that becomes a symbol of their connection. Eun-joo later encounters her former fiancé at work; they had planned to marry, but he moved abroad and she stayed in Korea. She asks Sung-hyun to intervene in the past to prevent the breakup, a request that tears at him as he contemplates sacrificing his own happiness for hers. After long reflection, he chooses to set aside his own desires and writes back to Eun-joo, wishing both her fiancé and herself luck in the future.
The story reaches a dramatic turn when Eun-joo visits Sung-hyun’s school and learns from a friend that Sung-hyun died in a traffic accident while meeting a friend two years earlier. She realizes that on the day she last saw her fiancé before they separated, she witnessed a car crash in which Sung-hyun was the pedestrian. The house being built on the beach was his design, meant for her, and she races to the mailbox to plead with him not to go. The two come to understand that every choice ripples across time, shaping the paths they must follow.
In the film’s haunting final beat, we return to the moment when Eun-joo is about to drop her Christmas card into the Il Mare mailbox. A figure approaches with a letter in hand—the letter she had sent to warn Sung-hyun. He did receive her warning, chose not to intervene, and avoided the fatal accident. With that revelation, Eun-joo and Sung-hyun finally manage to meet, and their two timelines converge in a hopeful, long-awaited reunion.
