Along with his young son, Ji-ho, Woo-jin misses his wife Soo-a, who died after promising to return a year later with the rainy season. Miraculously, they reunite with Soo-a when the rainy season comes around, but she has no memory of her husband and son whom she dearly loved.

On the eve of the dreaded university entrance examinations, the lives of 24 of the smartest and brightest students at one high school, are on the line. Locked inside a classroom and nowhere to escape, a television monitor broadcasts a terrified Hae-yeong, the top student at the high school, tied up and seated on a chair inside an aquarium that slowly begins to fill with water. A voice from the monitor announces if the 24 cannot finish the exam administered within the allotted time given, Hae-yeong will die and so will everyone held captive inside the classroom. Thus begins the examination that will determine whether they live or die.