A model high school student who's steeped in a world of serious crime finds his double life upended when a classmate takes an interest in his secret.

It is a story of high school students who become tangled in a series of conflicts and events that challenge human values and morality. Jisoo (Kim Dong-Hee) is a model high school student. To pay for his university tuition fee, he makes a bad choice. This leads to a serious crime that takes place. Minhee (Jung Da-Bin) and Gyuri (Park Joo-Hyun) attend the same high school as Jisoo and they get involved in Jisoo's crime.

Extracurricular portrays the lives of Highschool students Choi Jisoo and Bae Gyuri as they make choices that lead to a "spiralling down the rabbit hole" effect. Jisoo had been a model Highschool student that rose from a terrible background which included his dead beat father and barely any financial stability. Gyuri on the other hand was the daughter from a CEO of an entertainment company. The show incorporates multiple people from different age groups but mainly surrounds Jisoo and his "business".