Kim Su-ho Cha Tae-hyun returns to his hometown after a decade to attend a high school reunion in modern-day Korea. Along the walk, a female voice seems to call his name, echoing in his head as he moves through familiar streets. His old friends doubt his presence, noting that he hasn’t fully moved on from a girl and that the reunion will mark the anniversary of Bae Soo-eun Song Hye-kyo’s death. To everyone’s surprise, Su-ho shows up, and after the celebratory moment, they drift toward a lighthouse where a sharp remark about Soo-eun unsettles him, sending him into tears. In a flashback, a younger Su-ho is rescued from drowning by a pretty girl who loses her pager in the process but leaves before the rescue is complete; Su-ho wakes believing his friends were the ones who saved him.
In class, Su-ho is drawn to Soo-eun as she stares often in his direction. He notices her, and when she asks him to buy her a croquette, their exchange quickly shifts from casual to intimate. They eat together in defiance of the gossiping onlookers, and the moment they share makes it clear that they are seen as a couple by their peers. The captain of the judo club, who is interested in Soo-eun, confronts Su-ho, and a tense escalation follows. Soo-eun steps in, confirming their relationship, and the captain roughs Su-ho up. Su-ho’s friends intervene, and they escape on bicycles, the mood between them thick with new, fragile trust.
Soon after, near the lighthouse, Soo-eun hints she saved him long ago, a detail he misreads as a simple gesture rather than the truth of their intertwined destinies. They watch the sunset together, deepen their bond, and on the way home she slips him her number in a moment that feels both ordinary and fated. That night, they confess their feelings for one another, marking the beginning of something powerful and fragile.
The next day brings a sudden summons: Su-ho’s grandfather has collapsed. He rushes to his grandfather’s side only to discover he’s fine, having feigned illness to speak with him privately. Soo-eun arrives with Su-ho’s bag, and the moment widens to reveal a long, personal history. Su-ho’s grandfather shares his own love story—how he joined the army, was separated from his first love Soon-im Moon Jeong-hee, and left her a necklace as a keepsake. Years later, he becomes an undertaker, arranging a funeral for Soon-im’s husband and eventually marrying another woman, Su-ho’s grandmother, in the process. This family memory unsettles and also clarifies for Su-ho the depth of his own feelings for Soo-eun. He writes her a note confessing his heart, not realizing she stands behind him; she softly answers, “Me too,” and the couple becomes officially together.
A plan for an overnight stay on Fog Island is proposed by Su-ho’s friends, who orchestrate the trip to give the couple space. At the pier, Su-ho discovers their scheme, but the chance to be alone with Soo-eun on the island strengthens their bond as they share their first kiss. The next day, Soo-eun suddenly faints and she is rushed to the hospital, where she is diagnosed with terminal leukemia. Her last wish is to return to their island, a request that adds a poignant urgency to their love.
Meanwhile, Soon-im’s own last wish had been to be reunited with Su-ho’s grandfather, and the grandfather’s memory of that reunion provides a quiet, resonant parallel to the young lovers’ fate. While Su-ho visits Soo-eun in the hospital, she expresses her deepest wish to see the sea again. He reveals two ferry tickets, but a typhoon shuts down all ferries, blocking their passage. Su-ho pleads with authorities to grant them passage, but the storm and the demand for time test their resolve. In the tense moment, Soo-eun collapses, leaving Su-ho to confront an impossible choice.
Back in the present, Su-ho returns to Fog Island and finds Soo-eun’s bag. A diary entry reveals that she planted seeds on a hill during their earlier visit. He goes to the hill and discovers it covered in purple flowers, a living, final gift from Soo-eun that stands as a testament to their love and to the memory of Soon-im and the grandfather’s enduring history.
