The manga takes place during a summer holiday Onizuka had while he teach Kanzaki Urumi's class. It started with Onizuka going back to his hometown, Shonan, for the holiday. Shonan is his base back during the day where he was known as the Onibaku (along with his best friend, Danma Ryuuji).
There, Onizuka met with a girl who works in an Orphanage like place, but instead of for kids with no parents, it was for kids with problematic parents. There, Onizuka helped the kids with their problems.
It ended with Onizuka chasing after the Mayor of the town after the Mayor's act caused one of the kids (a girl) with serious burn injury. With the help of his Onibaku underlings, he successfully caught the Mayor in a bus full of cabaret girls after posing as a terrorist bus hijacker. He then forced the Mayor to resign his office, apologize to the injured kid, and take care of her medical bills, including a plastic surgery to fix her damaged face.
At the end it was shown that Onizuka has returned to the school, with his students doubting his story (about his holiday) and then surprised to find that Onizuka didn't make up the story after one of the caretaker girls called him.