Na Doo-su, Lee Seung-hyeon, nicknamed Yalkae (a cheeky, cocky person), is a second-year student at a Christian missionary high school who often acts like a troublemaker because of his rocky academic record. His everyday routine is upended when a new teacher arrives: Baek Sang-do, Kim Jung-hoon, a mentor who was once the father of Na Doo-su’s apprentice.
Na Doo-su, while running errands for Baek, finds his heart drawn to Insook, a convenient-store clerk and a Moran Girls High School student. In his bid to connect with her, he joins a biking club, hoping it will bring them closer together. Meanwhile, the Na family schemes to marry off Na Doo-su’s older sister, Na Du-ju, to Baek. Baek works as a private tutor for Na Doo-su, and through those tutoring sessions, Na Du-ju starts talking with him, adding another layer to the evolving dynamics.
Baek encourages Insook to keep a respectful distance from Du-su in the name of his academic improvement, setting up a quiet tension between affection and obligation. The tension thickens when Du-su pulls a prank on Hocheol, a diligent though poor student, by painting his glasses red, which leads to the glasses breaking. Baek’s anger flares, and he contemplates removing Du-su from the school, but a later teachers’ meeting ends with Baek pleading for forgiveness and saving Du-su from expulsion.
The following day, Hocheol is absent from class. Du-su goes to his home and finds Hocheol with a wounded leg, the result of a bike ride gone wrong due to poor eyesight. Feeling guilty, Du-su apologizes, and Hocheol responds with a powerful message about a brighter future. Inspired, Du-su decides to help by delivering milk to earn money, and as he does, his own grades begin to improve.
Du-su learns from Hocheol’s neighbor that the condition of Hocheol’s leg has worsened, and he makes a trip to the hospital to gift him a pair of new glasses. While delivering milk, Du-su himself is hurt in a traffic accident, yet his concern for Hocheol remains unwavering. The story also highlights the caring influence of Principal Richard Hudson and the support of Du-su’s family and friends.
In the end, the film resolves with a hopeful, celebratory note: Du-su and Insook’s relationship is officially acknowledged, and Baek’s betrothal to Na Du-ju is celebrated, bringing a warm sense of closure to their intertwined lives.
