After joining the school's elite secret society, Eva gets curious on the secrets the Fives are hiding about the school. Sixty years earlier, Connie and five other students volunteer to test the school's nuclear fallout shelter. However, Connie and the other students quickly discover there is more to the test than they thought, as they are forced to follow a series of commands that start to get deadly. Separated by sixty years, Eva and Connie's stories become intertwined as Eva discovers the mystery of how six students went into the fallout shelter, but only five came out.