This is actually a sequel to GTO and GTO Shonan 14 days with a more mature Onizuka and a little bit bloodshed story.
This series actually has two events. The first part is a flashback before he got in prison and the second part is the present. Onizuka is story-telling it piece by piece to his co-inmates, even now he haven't told them the part how he got there. Soon, the flashback story-telling will connect with the current event happening in the prison and revelations of the culprit behind his imprisonment.