Shuugo Hayami is a doctor at a nursing hospital, which was once a psychiatric hospital. One evening, when he was on night duty, a man in a clown mask breaks into the hospital, and he is not alone. He is with Hitomi Kawasaki, a university student who has been shot by the masked man. The masked man tells Shuugo to treat Hitomi, and also takes the 64 people in the hospital hostage. Soon, Shuugo and Hitomi try to escape from the hospital, and they encounter mysterious cases one at a time.

Kamen Byoutou is an adaptation of Mikito Chinen's 2014 novel of the same title.