Seijiro Matsuzaka is a member of parliament engaged in a decade-long corrupt partnership with Keiichi Suzuki, president of Ōshika Construction. This scheme permitted Ōshika Construction to utilize substandard materials and reduced labor in projects while securing regulatory approval. In exchange, Suzuki provided Matsuzaka with financial kickbacks, embezzling funds meant for proper construction standards. Matsuzaka secured the arrangement by recruiting the Overlord Black Dragon Gang, a yakuza group, to provide security and intimidate potential whistleblowers or investigators.

The illicit activities drew the attention of the Government Crime Investigation Agency (GCIA), an independent entity authorized by the Japanese government to combat high-level corruption. GCIA investigator Jotaro Zaizen was assigned to uncover the conspiracy. Matsuzaka's role positioned him as a key figure within a broader network of political and business corruption. His involvement centers on facilitating the corrupt partnership between Ōshika Construction and political interests.

As the GCIA investigation advanced, it connected Matsuzaka's actions to higher echelons of power, including Hakudo Imazu, a shadowy figure controlling political and business operations in Japan for decades. The scope of Matsuzaka's influence and the repercussions of his exposure remain tied to the central corruption case, without documented resolution of his character arc.