In a high-rise building in downtown Seoul, there is a mass outbreak of an unknown infection. Within no time, the building is sealed off, and the people inside are trapped.

The infected, after first crawling on all fours like beasts, gradually evolve and begin moving on two feet. Before long they start to identify the survivors and make coordinated group attacks.

Biotechnology professor Kwon Se-jeong and several other survivors manage to find Suh Young-chul, who claims to have injected himself with a vaccine, and head to the roof where a rescue team is waiting.

However, as they make their way upwards, the situation becomes ever more precarious, and Suh Young-chul uses the infected to block the group’s path…