The film tells the love story of a male engineering college student, Gyeon-woo, and “the Girl” (who is never named in the movie). Gyeon-woo just cannot seem to catch a romantic break. One day, at dinner and drinks with his college friends, Gyeon-woo is interrupted by a call from his mother, telling him to visit his aunt and meet a potential date. At the train station on his way to his aunt’s, he observes a drunk girl, standing precariously close to the edge of the train platform as the train approaches; he pulls her to safety just in time. Inside the train, Gyeon-woo cannot help but stare at the girl who is his “type” but repulsed by her drunkenness. Finally, she throws up on a passenger and faints but not before she calls Gyeon-woo “honey”. The passenger aggressively chides Gyeon-woo and tells him to take care of his “girlfriend”. Gyeon-woo, completely flustered, leaves her on a subway platform bench, but his conscience compels him to take her to the nearest hotel for safety. While the girl is passed out on the bed, her phone rings and Gyeon-woo picks up. He gives vague answers in regards to the girl’s whereabouts and perhaps through GPS tracking, police raid the room and Gyeon-woo gets maced and sent to jail. He is sent home from jail the next morning, and to his surprise he gets a phone-call from the girl, who demands they meet by the train station so she can figure out what happened the night before. Over soju the Girl cries, admits to breaking up with her boyfriend the day before and gets thoroughly drunk, resulting in a second trip to the same hotel. Thus begins his comically ill-fated relationship with the Girl.
After this second overnight stay at the hotel, she begins to become a more active part of his life. She visits Gyeon-woo in school and pulls him out of class, telling the teacher that Gyeon-woo is the father of her soon-to-be-aborted baby. The Girl’s mood swings wildly from joyful to downright violent, but Gyeon-woo puts up with it and lets her abuse him for her amusement.
She is an aspiring scriptwriter and throughout the movie gives Gyeon-woo three different screenplays from different genres. The first is an action movie—The Demolition Terminator—which switches gender roles, symbolically having the Girl save her helpless lover (Gyeon-woo). The second is a wild perversion of a Korean short story—Sonagi—in which the Girl, having died, asks that her lover be buried along with her—even though he’s still alive. The last is a wuxia/samurai movie spoof full of genre clichés and anachronisms. All three feature the same common thread: the Girl is from the future.
Despite all the horrible things Gyeon-woo endures, he is determined to help cure the girl’s pain. He decides to surprise her on her birthday and takes her on a nighttime trip to an amusement park which ends up quite differently from how he planned: the pair encounter an AWOL soldier who holds them hostage and rants about his misery after being jilted. Gyeon-woo convinces him to release her, and she in turn convinces the soldier to free Gyeon-woo and go on with his life and pursue another love.
The Girl and Gyeon-woo’s relationship takes a turn for the better; her cruel treatment toward’s Gyeon-woo is her way of showing affection and the two grow closer. After a day of hanging out, he walks her home in the pouring rain and she demands he meet her father, who is a habitual drinker. Her parents do not take to Gyeon-woo and on leaving, he overhears an impassioned argument between the girl and her mother over her relationship with him. He does not hear from her for quite some time and his life without her begins.
One day however, the Girl calls him and tells him to bring her a rose during class (the Girl attends an all-girls college) to commemorate their 100th-day anniversary. He does this, leading to a touching and romantic scene where he arrives in disguise as a food delivery person into a packed auditorium and watches her play the melody of George Winston’s variations on Pachelbel’s Canon in D on a piano onstage. He presents her the rose and the two hug while the classmates applaud in approval at his romantic gesture. As part of their celebration, they hit the bars and clubs together dressed in high-school uniforms; she gets drunk and as Gyeon-woo carries her on his back, a stranger slips him a condom. Gyeon-woo drops her off inside and is confronted at her house by her parents again; the father interrogates him and forces him to empty his pockets, where he embarrassingly presents the condom. Her father demands the two to break up.
The Girl does not contact him again and Gyeon-woo naturally thinks they have broken up, until one day seemingly out of the blue, she calls Gyeon-woo to meet her for dinner as she is on a blind date. The Girl introduces Gyeon-woo to the date, who thinks Gyeon-woo is a great friend of hers based on how highly she speaks of him. She is unusually soft and gentle as there is clear awkwardness between the two. She excuses herself to the washroom and appears to not be handling the break-up so well. The Girl returns to the table to see Gyeon-woo gone; he left while she was in the bathroom, but not before offering the blind date ten rules to follow to ensure her happiness; only order coffee at cafes, prevent her from over-drinking, give in to her at every circumstance, when she hits you pretend it hurts when it doesn’t and vice versa, surprise her with roses, learn Kendo and Squash (sport), and more. While the blind date recites these rules to her, she realizes how well Gyeon-woo understands her and realizes her love for him. She abruptly leaves her date and searches for Gyeon-woo at the subway station.
They cross paths several times, but never run into each other, so she goes into the subway security office to see the security monitors. She sees him standing close to the edge of the platform (just like she was in the beginning). While an employee makes an announcement in the microphone, she yells his name; he hears and they re-unite at the office.
Once reunited the two realize they are at a turning point in their relationship, but, for some unspeakable reason, the Girl decides it is time for them to part. As a gesture to their happy times the two write letters to each other and bury them in a “time capsule” under a particular tree on a mountain in the countryside. They agree to meet again at the tree after two years to read the letters together. After burying the “time capsule” they go their separate ways for good.
During the two-year span, Gyeon-woo works hard to improve himself; he practices Kendo and Squash and learns to swim. To pass the time, he also began writing about his dating experiences with the Girl on the internet as a means to cope. His stories gain so much notoriety that he is approached by movie producers to turn his letters into a movie; he is ecstatic because the Girl’s life-long dream was to have one of her screenplays made into a film. He so desperately wants to share this news with her, but chooses to wait until they agree to meet.
Two years have passed and on the agreed date, he travels to the tree on the mountain-top, but the Girl does not show up. He comes day after day without opening the time-capsule in hopes that she would appear, but she never does. Eventually, he opens the time capsule and reads her letter and learns the root of her angst and behavior: Gyeon-woo reminds her of her previous boyfriend who, rather than breaking up with her, actually died before she met Gyeon-woo. On the day they met on the train platform, she was supposed to go on a blind date set up by the mother of her deceased ex-boyfriend, with whom she kept a close bond with. Because of her chance meeting with Gyeon-woo, part of her feels that it was her ex that brought them two together and she develops guilt for falling in love with him. She needs time apart to heal and move on from her ex alone.
A year after Gyeon-woo visits the tree, the Girl finally arrives. Sitting under the tree is an old man. She reveals that she was supposed to meet someone at the tree a year ago, but lacked the courage to do so. She believed that if she was truly meant to be with him, destiny would bring them together. During their conversation the old man reveals the secret of the tree, that it is not the same tree; the original tree had been struck and killed by lightning a year before and a similar tree had been planted by a young man so that his special someone would not be sad. The tree was dead and split into two. After the girl reads his letter, she tries to call Gyeon-woo repeatedly, but is unable to contact him.
Some time later, the Girl is on the train and the door closes right before a man tries to board. She has her back turned to him, so she doesn’t realize it is Gyeon-woo, who realizes it is her and chases it down the platform to no avail. They fail to cross paths again.
As it turns out, the Girl is on her way to lunch with her deceased boyfriend’s mother, where she plans on introducing her to her nephew for a blind date. The nephew arrives and it turns out to be Gyeon-woo; as it turns out, Gyeon-woo is the boy that she had been trying to introduce to the Girl for years. The mother offers reasons as to why they’d be a great match for one another, but Gyeon-woo and the Girl are too busy gazing into each other’s eyes. The mother asks “Do you two know each other?”; the camera pans out and they are holding hands under the table.