Originally Posted by wikipedia

An inscrutable and abnormal territory, known as Hell's Gate, appeared in Tokyo and altered the sky. At the same time, people who possess various special abilities emerged. The stars disappeared, replaced by fake stars which correspond to those people with special powers. However, the cost of gaining those abilities is the loss of human emotion. Kept secret from the knowledge of the masses, these individuals able to murder in cold blood are known as Contractors. Various nations around the world use Contractors as spies and agents, often resulting in violent battles for information.

Hei - the main character in the story - is sent to Tokyo under the disguise of an ordinary Chinese student, Li Shenshun (李舜生, Ri Shunsheng?), but is really a Contractor working for an unnamed organization. His Messier code, or star number, is BK201. Hei fights against other Contractors with unpredictable abilities while carrying out missions and gathering information for his Agency.