On his way back to school after a routine medical check-up, an anxious Minjae has a brief encounter with a stranger, Youjin, on the subway. They attend the same university but part ways without knowing each other's names. Meanwhile, exhausted by their research, Minjae and his lab partners create bucket lists to regain their energy. Pretending to be indifferent, Minjae secretly adds "start dating" to his list. He resolves to confess his feelings to his long-time crush. However, just as he is about to do so, he runs into Youjin and gets swept up in an unexpected incident.

Youjin resolves to pursue a master's degree in robotics and asks Minjae for help, but Minjae considers it a reckless choice. To make matters worse, his leg begins showing signs of trouble. While touring the lab thanks to Minjae's lab partners, Youjin causes a major accident. Overwhelmed by anxiety that he has desperately tried to suppress, Minjae finally snaps. Despite his strong opposition, Youjin becomes captivated by the world of robotics. Can Youjin successfully make her way into the robotics lab?

From her very first day at the robotics lab, Youjin constantly clashes with her peer, Sejin. Sidelined with no real responsibilities, she grows anxious that she is falling behind. Although Minjae is conscious of Youjin after what happened on the night of the welcome party, he intentionally keeps his distance. To Youjin, however, his behavior feels cold and indifferent. Her pent-up frustration eventually boils over. Meanwhile, Seohyun, an original lab member and Minjae's first love, leads the entire group to a place called "Dieland."

As rumors spread that Minjae was seen with a mysterious woman, a jealous Youjin makes a bold move.

Bonding over their shared pain, Minjae and Youjin quickly grow close. But when Youjin's brother, Youseong, senses something suspicious, he starts investigating. As Youjin draws closer, Minjae becomes frightened and starts questioning whether he has the right to love someone and whether he should continue their relationship. While Minjae keeps trying to pull away, Youjin boldly moves closer. On a rainy night, Minjae can no longer ignore Youjin's feelings or his own.

Minjae and Youjin start a secret romance. From day one, the gap between them only grows.

Minjae and Youjin help Sejin pursue Subin. Youjin's test of love triggers Minjae's old insecurities.