Twenty Five Twenty One - Ending Explained

The series finale reveals that Na Hee-do and Baek Yi-jin do not end up together. Their relationship dissolves because the distance—both physical (New York to Seoul) and emotional (Yi-jin's trauma from reporting on the 9/11 attacks vs. Hee-do's need for emotional support)—becomes a burden they can no longer carry. The 'father' of Hee-do's daughter, Kim Min-chae, is never shown and is explicitly not Yi-jin. The final twist is that the Adult Hee-do has forgotten many of the specifics of her youth, and it is only through her daughter's discovery and returning of the diary (which Yi-jin had tried to return years earlier) that she finally experiences closure and thanks the 'past' Yi-jin for the love they shared. This reveals the title's meaning: 25 and 21 were the ages when they finally broke apart and became truly independent adults.

While many viewers interpret the ending as a tragic failure of love, an alternative reading suggests the ending is a triumph of self-actualization. In this view, the breakup wasn't a failure, but a necessary step for both Hee-do and Yi-jin to become the successful adults they are in the present. Another interpretation focuses on the 'Unseen Father' of Min-chae; some fans argue that by never showing the husband, the creators emphasize that the romance with Yi-jin was the definitive emotional peak of Hee-do's life, regardless of her eventual marital status.