Ten years ago, an inscrutable and abnormal territory known as Hell's Gate appeared in Tokyo, altering the sky and wreaking havoc on the landscape. The heavenly bodies disappeared, replaced by false stars. During this time, people who possess various special abilities emerged. Kept secret from the masses, these individuals, known as Contractors, are able to murder in cold blood. Various nations and organizations around the world use Contractors as spies and assassins, often resulting in violent battles for information. Each fake star has a corresponding Contractor, and they vibrate when their Contractor's power is being used. As such, Contractors are usually identified by their star's Messier Catalogue Number. However, their abilities are gained at the cost of their humanity. They are implied to lack human emotions which includes no aversions to murder, and they are regarded as extremely rational and logical in their decision making. Contractors are thus named because of their need to always complete a remuneration or payment each time their power is used. It is not entirely clear what happens to one if he or she fails to complete the remuneration. Contractors are described as being psychologically compelled to complete their remunerative task.