The Mole Song: Undercover Agent Reiji (2013)

Reiji Kikukawa, who has a strong sense of justice, graduated from the police academy with the lowest score ever. He becomes a police constable, but is suddenly fired by the Police Chief due to "disciplinary" issues. In actuality, the firing is part of a carefully orchestrated plan. Reiji is ordered to become a "mole," an undercover cop. His target is Shuho Todoroki, the boss of the Sukiyakai gang. The group is the largest crime group in the Kanto area. Masaya Hiura, who works as a young boss of a Sukiyakai affiliated gang likes Reiji. While going through various hardships, Reiji works his way towards Shuho Todoroki.