Sanji Sumida is a police officer at the headquarters where Miyanomori Susumu serves and a relative of Susumu's mother, Sumida Sanae. Initially assigned to traffic police, Sanji chafes at the role, viewing it as offering limited value regardless of effort. He perceives Susumu's position as a Miyanomori heir as distinctly advantageous, underscoring their differing statuses. Seeking greater significance, Sanji pursues and secures a transfer to the Tokubetsu Koutou Keisatsu (Tokkou), a special police force investigating political threats. This career shift alters his demeanor and uniform, noted by figures like Haru Asagi.

Susumu questions Sanji's motives, suspecting external influence behind his Tokkou application, but Sanji insists his decisions are self-driven, rejecting any inherent benefit from his Sumida connection. He becomes entwined in political machinations, colluding with Kawamura, a politician aware of Mamoru Mimori's existence and grudge against the Miyanomori. Kawamura facilitates Sanji's Tokkou placement, and Sanji reciprocates by requesting Kawamura assign Sanae Sumida to the Tokkou, deepening his involvement in power struggles.

Sanji's actions directly facilitate the assassination of Prime Minister Saeki on September 25, orchestrated by Mamoru Mimori. He provides Mamoru with a written invitation under the alias "Itou Toshi" to access the Sumida Ball, enabling reconnaissance on the Miyanomori brothers. During the event, Sanji expresses concern about Mamoru's presence but is dismissed. His Tokkou affiliation and political alignments culminate in his role enabling the prime minister's assassination.