In the early 21st century, a devastating act of bioterrorism has drastically reduced the global population. Humanity is now divided into two distinct groups. The first group, known as the Nokusu, consists of individuals who contracted a virus but survived, developing antibodies as a result. These people are generally young, healthy, and possess high intelligence, but they are extremely sensitive to ultraviolet light, leading them to be more active at night. The second group, the Kyurio, are those who managed to avoid contracting the virus altogether. The Nokusu tend to discriminate against the Kyurio. Tetsuhiko Okudera is a member of the Kyurio, born into a poor village. Dissatisfied with his status as a Kyurio, he longs to be one of the Nokusu. His childhood friend, Yui Ikuta, is dedicated to revitalizing their impoverished village.