Full spoilers follow.
The buried crime. Noh Eul's father, a street food seller, is killed in a hit-and-run caused by Yoon Jeong-eun's daughter. Rather than face it, Jeong-eun bribes the prosecutor in charge, Choi Hyun-joon, to switch the driver and indict a different man. The case closes in her favor. This single act is the origin of everything: Noh Eul's poverty, her guardianship of her brother, and the separation of the two leads.
The parentage twist. Choi Hyun-joon is Shin Joon-young's father. The man who buried the case that destroyed Noh Eul's family produced the man she loved and lost — the Romeo-and-Juliet configuration the drama builds its second half on. Retroactively, the teenage separation stops looking like bad luck and becomes the direct consequence of a purchased verdict.
The false identity. Choi Ji-tae, the prosecutor's other son, befriends Noh Eul and her younger brother while posing as a poor graduate student named Hyun-woo, precisely because he knows she would never date the son of the prosecutor who covered up her father's case. His deception runs in parallel to Joon-young's, and both are eventually exposed.
Noh Eul's counter-scheme. Once she understands who is who, Noh Eul calmly asks Ji-tae whether dating him would make his parents' world crumble, then proposes that they date — not affection but a revenge plan aimed at the family that bought its way out of her father's death.
The diagnosis. Joon-young has cancer. It is present from the beginning and explains the entire architecture of his behavior: the manufactured reunion, the documentary he permits, the cruelty he uses to keep Noh Eul at a distance. On rewatch nearly every early scene inverts, which is the show's chief structural payoff.
The confession. Choi Hyun-joon turns himself in without legal representation, accepting full punishment for the cover-up. The justice plot resolves before the romance does, deliberately clearing the field so the finale can be about mortality alone.
The finale. In episode 20, Joon-young storms out of Noh Eul's hospital room in anger, then checks her phone and finds a video of her giving an interview for his documentary. In it she says that one day no amount of patience will be enough to win against cruel fate, and that a day will come when she will not have to wait for him anymore — the confession neither of them could make face to face, delivered through the very film he permitted about himself. In his final hours his memory fractures but clears long enough for him to tell his mother that he tried his best to live well. He then dies quietly on the patio, falling asleep beside Noh Eul with his head on her shoulder. There is no remission, no reversal, no last-minute reprieve.
The meaning that only lands at the end. The documentary was never going to capture Joon-young; it was always the container for Noh Eul's testimony. And the 'second chance' the reunion appeared to offer was never usable — it was a chance to be witnessed, not to be saved.