Class 3-2 of Sungdong High School was just like any other class: a mismatched group of individuals preoccupied with their crushes, upcoming university exams, and other ordinary day-to-day worries. Yet, when strange purple orbs rain down from the sky one day, the students find their lives changing beyond the limits of imagination.

Capable of sporadic and sudden destruction, these spheres quickly send the world into a panic. With the South Korean military forces stretched thin, the government hastily enlists those in secondary education and above into the army. Now drafted into military training, Class 3-2 must abruptly set down their pens and pick up rifles instead.

Promised that they will encounter little danger—and that bonus university points await them post-service—the reluctant students are coaxed into accepting their new duty. Marching onward with an apathetic and easygoing attitude, the class begins their military service, unaware of the grim reality that will soon erode their innocent, carefree spirits.