From MangaHelpers:

Since his girlfriend was beaten to death by a biker gang, Yamato Souhei has driven himself to become stronger and better at fighting. He's newly out of juvenile detention, and his friend Kijima helps him get back into life with a new job. His strength will be useful for the bouncer organization "Back Drop," which will make use of his abilities to provide security for clubs and other places.

Before he even signs on with the company, though, he happens across a group of people beating a man to death. Yamato can't save the man, but he does see the face of one of the attackers. Along with his new job as a bouncer, it looks like Yamato has become involved in a dangerous cycle of violence. [tethysdust]