One day, a mysterious virus spreads through the city of Seoul in the form of mutant cells that keep multiplying. When people come into contact with these cells, they turn into hideous and dripping creatures that can infect others at any time.

Yeon-U, who is on a school trip to Seoul, is excited at the thought of getting closer to So-Yeon, a classmate whom he had a crush on. But for Yeon-U and his classmates, the trip soon turns into a nightmare. In order to survive, these high school students must find a way to escape the city before it is completely engulfed by the virus.