21: Two One is an erotic visual novel adventure game.

Takuya Kirishima is a brilliant young surgeon who can save anyone with a disease. He spends his days working with his childhood friend Seri Sasaki, who is a nurse in the same hospital. One day, a murder is commited in the hospital and people starts suffering a strange disease not seen before. Takuya starts looking for the answer of these strange happenings so he can protect Seri.