Knight Hunters is about these four men, Omi Tsukiyono, Ken Hidaka, Yohji Kudoh, and Aya Fujimiya, are a group of assassins called "Weiß." They cover their operations by working at a flower shop during the day and the basement is their hideout. This is no ordinary group: they kill the murderers, big businessmen, and other assassins that the law cannot touch.
Each one is fighting for their own reasons, whether it is revenge or to help a loved one. But they soon realize that there are connections between some of their cases and try to find out what is really going on. They use whatever they can to complete their missions and kill the people that are targeted by Weiß's leader.