After a year of living peacefully with alien spheres in the sky, the seniors of Sungjin High School worry about their CSAT. However, one of the spheres drops on a military camp as their exams are nearing.

After the spheres start to drop, the students undergo a strict military training and chaos ensues at the shooting range. Later, one of the spheres kills a student.

To avoid panic, the news of two people from Class Two dying are kept a secret. Soon, Class Two students grow tired of the training and try to escape.

The sphere attacks the vehicle, leaving So-yeon severely injured as other students seek shelter at a construction site. Later, Lieutenant Lee attempts to save the students.

Chun-ho tries his best to get the kids to return to the camp safely. Meanwhile, the kids start to bond with each other as they learn more about being soldiers.

The survivors of the encounter with the spheres are heavily traumatised and struggle to cope with reality, which makes the group consider abandoning the mission for their safety.