The story of 20th Century Girl revolves around Na Bo-ra, a seventeen-year-old who is devoted to Yeon-du, her best friend. Yeon-du, however, is heading to the US shortly to undergo a particular form of cardiac surgery. After doing a nice deed for her, Yeon-du falls in love with Baek Hyun-jin, and her buddy leaves Bo-ra with instructions.
Bo-ra now has the responsibility of learning Hyun-jin's identity, keeping her best friend updated on all of his activities, and ensuring that Yeon-du will have a partner in the works when the time comes. And Bo-ra acts accordingly.
Bo-ra starts following Hyun-jin around the school and discovers that he is bothersome. To do so, she befriends him and joins the university Broadcasting Club, where she also meets Woon-ho, another buddy of his.
Bo-ra is being cruelly teased by Woon-ho for what appears to be a small-time crush on Hyun-jin. She is even forced to steal an adult film from her family's store so that he might watch it. But throughout their time together, Bo-ra and Woon-ho develop a romantic relationship.
When Yeon-du comes back, she realizes her error and admits that she was lusting for Woon-ho rather than Hyun-jin.
She thought Woon-ho, who had recently moved overseas from New Zealand, was Hyun-jin that day because he was wearing his name on the school uniform. Heartbroken, Bo-ra decides to delete the email where she confesses to her friend that she was in love with Woon-ho by hacking into her friend's account.
And so the movie's angsty drama portion begins. Bo-ra, who is obviously in love with Woon-ho, ends up crying a lot for him throughout, even though she acts as though she doesn't like him anymore.
Yeon-du can't keep this a secret for long, so when she learns of her friend's crush, she sets up some alone time for the two of them.
But Woon-ho needs to go back to New Zealand because his family is there, so it's too late now. Before he departs, they manage to make amends and agree to meet up at a prominent Korean institution for education. However, shortly after the year 2000 arrives, Woon-ho ceases answering Bo-ra's emails. As soon as she arrives at the school where they had agreed to attend together, she starts stalking him around the grounds.
As technology advances, things gradually alter. She is seen in numerous sequences attempting to move on from him, including going on blind dates, but she always breaks down in tears because she just can't let go of Woon-ho. His younger brother, who is now an artist, doesn't tell her the truth about Woon-ho's passing until many years later.
That’s why he didn’t respond to any of her emails.
