With a nasty attitude and something rotten in his soul, high school student Yu Seongbin has picked the weak Han Gyuhwan as his victim. As no one cares enough to put a stop to the violence Gyuhwan suffers, his mind festers with fantasies of standing up to his bully and the useless adults around him, picturing himself pulling out a shotgun to exterminate them all.

When their class leaves for a trip, it quickly spirals into a living nightmare for Gyuhwan. Seongbin inevitably starts harassing him, but Gyuhwan finally finds the courage to resist him. However, his bravery is short-lived, and he escapes into the woods, with an enraged Seongbin close behind him. But after he falls off a cliff, Gyuhwan stumbles upon a strange shotgun and a box of 30 bullets that he, in his desperation and exhaustion, intends for his classmates and apathetic teacher.

Back at the camp, the class' talent show turns sinister. Something unnatural creeps in from the shadows, and as people begin transforming into twisted, bloodthirsty monsters, it seems that only Gyuhwan's mysterious shotgun can truly finish them off.