Citizens from upscale Gangnam in Seoul start experiencing unusual and terrifying symptoms, devolving into inhuman creatures, leaving only a few survivors with the possibility to make it out alive.

The film revolves around the relationship between two high school students, Yoo Shi-Eun and Min Hyo-Shin. As the two girls become romantically involved, their taboo friendship causes them to be marginalized by the other students. Unable to cope with the social pressures of having a lover of the same gender, Shi-Eun tries to distance herself from the increasingly dependent Hyo-Shin.

Se-hee is haunted by her step sister Tae-yeon who used to be a promising swimmer but suddenly killed herself in a swimming pool 2 years ago. During the summer vacation, she joins the study camp at school for upcoming college entrance exam with other 30 elite students. On the first night, when Se-hee finds a riddling passage scribbled in the desk, she soon sees a dead girl’s body hung upside down. Then the TV monitors are on and the students watch another friend crushed by his bike in the corridor. Since then, the succession of cruel killings occurs whenever they fail to answer the given questions in time. Meanwhile, the hidden truth behind Tae-yeon’s suicide is slowly revealed and terrified students struggle to death to undo the puzzle before they become the next victim.

A staircase leading to the dormitory of a remote boarding school usually has 28 stairs, but every so often there appears to be 29. When someone steps on the mysterious extra stair, the horror begins.