A giant wall looms over Tokyo, shielding the city from a dangerous otherworld called the 'Hell's Gate'. Within the city, things are no less terrifying because Contractors, psychopathic killers with phenomenal powers, have started to appear. These killers are compelled to pay a price every time they use their powers, often in the form of a meaningless or painful task. As their deadly habits rack up a gruesome death toll, Kirihara Misaki and her team from the Foreign Affairs Public Security struggle to solve the cases and bring the Contractors under control. Their task is further confounded by the interference of a masked individual they title Messier Code BK201, a man with abilities that allow him to fight and defeat the Contractors. Who is this BK201? How can the Contractors be stopped permanently? And what does the appearance of the Hell's Gate mean for the people of Tokyo?