Kenzou Tenma, a Japanese brain surgeon in Germany, finds his life in utter turmoil after getting involved with a psychopath that was once a former patient.

In 2071, roughly fifty years after an accident with a hyperspace gateway made the Earth almost uninhabitable, humanity has colonized most of the rocky planets and moons of the Solar System. Amid a rising crime rate, the Inter Solar System Police (ISSP) set up a legalized contract system, in which registered bounty hunters (also referred to as "Cowboys") chase criminals and bring them in alive in return for a reward.