Yoon So-eun is a university student at Silla University who eagerly awaits the return of Dong-hee from his army service. Outside the amateur radio club, she reveals that HAM radio is her hobby, and he gratefully thanks her for all the letters she wrote him during the war. Visiting her friend Hur Sun-mi in the hospital, So-eun confesses that she is in love with [Dong-hee], setting a hopeful tone for a budding romance.
One night at home, a radio call comes through from another enthusiast, Ji-in. They discover they attend the same university and arrange to meet in front of the school clock tower, hoping to bridge the gap between their worlds. The plan becomes a puzzle of misaligned times: So-eun waits at an unfinished clock tower while Ji-in waits at a completed one, caught in a downpour. Their voices crackle through a radio with a disconnected power plug, and a startling truth emerges: she is a Junior in 1979 and he is a sophomore in 2000. At first he doubts the story, but, gradually, he becomes convinced that So-eun is telling the truth and that their conversations may be more than mere coincidence.
Meanwhile, So-eun’s relationship with Dong-hee continues to deepen, even as her time-crossing chats with Ji-in complicate her life. She shares her evolving feelings with Sun-mi at the hospital, though she cannot spend as much time with her friend as before. Ji-in also forms a close bond with Soe Hyeon-ji, a fellow student portrayed by Ha Ji-won, who seems to be always nearby, and whom Ji-in begins to care for more deeply. So-eun’s irritation with the radio conversations grows, but Ji-in starts offering glimpses of her future and his past, gradually convincing her that their connection might be real. She begins to speak about love and about Dong-hee, curious about what the future may hold.
The story intensifies during a period of student demonstrations when Dong-hee is injured. So-eun visits him in the hospital and signs her name on the plaster cast that encloses his injured arm. She also sees Sun-mi there, deepening the emotional web around her. That night, Ji-in reveals a devastating detail: his parents attended Silla University at the same time as So-eun, and their names—Sun-mi and Dong-hee—appear on his father’s plaster cast. This revelation shatters So-eun’s sense of reality as she rushes to the hospital to see the sleeping Dong-hee, now with Sun-mi’s name on the cast. The memory of Dong-hee and Sun-mi together glitches through her dream-filled mind and she feels a strange guilt she cannot explain. A frank conversation with Ji-in about destiny follows, and So-eun ends the romance with Dong-hee the next day, choosing a different path.
Later, Ji-in visits his parents’ country home. There, he discovers photographs of his parents from their college days, along with the plaster cast his father once wore and So-eun’s name written on it. He pores over the 1979 yearbook and realizes that the woman he has been speaking with every night was in love with his own father. This revelation disturbs him deeply as he wonders what might have happened if So-eun had chosen his father. He sets out to learn how So-eun is faring in 2000 and eventually finds her as a professor of English at another university. She remains unmarried but is clearly happy, and the sight of her stirs a rare nostalgia in him. He leaves with the intention of telling her what he saw, hoping to bridge the gap once more by radio — but the line is silent, and the signal dies.
In the end, Ji-in returns to his own life with his girlfriend Hyeon-ji, while So-eun continues to live and grow, coming to a quiet realization: Dong-hee was not her destination, but the cross-time connection has guided her toward a more personal understanding of fate, love, and the choices that define a life.
