Twenty-Five Twenty-One depicts a story from 1998 to 2021. In 1998, twenty-two and eighteen meet for the first time in an uneasy year as the whole world shook and called each others' name and when they became twenty-three and nineteen, they hurt each other. In the year of twenty-four and twenty, they relied on each other, and they loved only when they were twenty-five and twenty-one.
When the IMF financial crisis threatens to slash Na Hee-do's dreams, she sets out to get herself transferred to another school with a fencing program.
Though taken aback by Ko Yu-rim's cold attitude, Hee-do gives it her best in their first face-off. More of Back Yi-jin's situation comes into focus.
When a fortuitous opportunity presents itself, Hee-do begs her coach for extra training. An unanticipated visitor comes around hoping to see Yi-jin.
In exchange for a favor, Hee-do carries out a daily errand for Moon Ji-woong. She happens across a drunk Yi-jin after he fails another job interview.
Hee-do takes one giant leap closer to her dream. She's eager to tell Yi-jin the good news - but realizes he's disappeared without a trace.
Tensions run high between Hee-do and Yu-rim before an important event. Yi-jin takes on a new assignment and spots a familiar face in the crowd.
A frustrating media narrative dampens a monumental moment for Hee-do. Yi-jin chases down the only person who may be able to set things right.
Hee-do perceives a growing distance between herself and Yi-jin, even as he moves back into the neighborhood. Trouble at home adds to Yoo-rim's stress.
As Yu-rim's perspective on Hee-do shifts, so does her behavior. While Hee-do avoids seeing Yi-jin, he volunteers for a special documentary project.
Under the guise of work, Yi-jin takes Hee-do and Yu-rim on a weekend getaway to the ocean - and agrees to let Ji Seung-wan and Ji-woong tag along.
Hee-do grows upset when her career-obsessed mother fails to keep a promise. Yi-jin helps out Ji-woong's plans for a grand gesture at a music festival.
Unable to hold back from putting an end to a teacher's abusive behavior, Seung Wan stands her ground. Hee Do and Yu Rim support a distraught teammate.
Warnings about allowing his personal ties affect his work weight heavily on Yi-jin. The school fencing team enter their last competition together.
Shouldering the burdens of a sudden misfortune in her family, Yu-rim arrives at a decision that places her in hot water with the public.
Hee-do once again finds herself opposite her most formidable opponent. Yi-jin is busier than ever after transferring to another department.
In an emotionally charged exchange, Hee-do and Yi-jin confront the state of their relationship. Hee-do accidentally leaves her diary on a bus.