15 years prior to the beginning of the manga storyline, Danno Tatsuya, along with his childhood friend Ikuo Ryuzaki, witnessed death of his most important person- sensei. Since whole incident was covered up by the police force, Tatsuya and Ryuzaki swore to find the culprit and bring him to justice.
In the manga, 15 years after the incident, Danno Tatsuya is an important yakuza, working from inside the criminal world to find the ones guilty of sensei's murder.
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