Three years ago, Katsumi Yu traveled to Africa for a volunteer project as a doctor. However, a civil war broke out shortly after, and he was captured by a guerrilla group desperate for a doctor to treat the wounded. He continued doing what he had come to do—helping people in need—for several years. But with a change in the country's regime, the guerrilla group was hunted down, and Yu was forced into a kill-or-be-killed situation. He soon quit being a doctor and got a tattoo of Lucifer on his right arm as a reminder of his past crimes. Although Yu returned to Japan, he spent his days drinking. After a night of drunken brawls, he woke up in a clinic run by Dr. Minatono. This shady-looking clinic accepted all patients, even those without insurance or the ability to pay, yet still maintained state-of-the-art equipment. It turned out that Dr. Minatono dealt with clients, such as the mob, who could not go to regular hospitals for legal reasons, and in exchange received exorbitant amounts of money to help keep the clinic running. At first, Yu rejected the job offer from Dr. Minatono, but situations kept arising that kept him busy in the operating room.