Based on an action suspense shounen manga by Wakui Ken.
Watching the news, Hanagaki Takemichi learns that his girlfriend from way back in middle school, Tachibana Hinata, has died. The only girlfriend he ever had was just killed by a villainous group known as the Tokyo Manji Gang. He lives in a crappy apartment with thin walls, and his six-years-younger boss treats him like an idiot. Plus, he's a complete and total virgin... At the height of his rock-bottom life, he suddenly time-leaps twelve years back to his middle school days!! To save Hinata, and change the life he spent running away, hopeless part-timer Takemichi must aim for the top of Kanto's most sinister delinquent gang!!
Source: kodansha comics