After spheres from the sky drop to the ground, military training begins for the students to ensure their safety. However, the students find it too demanding and have a difficult time. As students head to the shooting range to learn how to shoot, they get caught off guard and fall into a crisis. Meanwhile, a student gets killed by one of the spheres.

For a year, people learn to live with the spheres without suffering any specific changes. Even with the alien-looking spheres floating all over the sky, the CSAT is what matters the most to seniors of Sungjin High School. When the exam is just around the corner, one of the spheres drops on a military camp for the first time, and more shocking news awaits them.