Takumi Mochizuki, an employee of Ōshika Construction, attempts to expose his company's corrupt dealings with politicians—including the use of substandard materials to maximize profits. This action places him in direct danger from his employer and the associated yakuza group, the Haoukokuryu-kai. Government Crime Investigation Agency (GCIA) agent Jotaro Zaizen rescues him from retaliation.

Following his rescue, Takumi joins Zaizen's team. He assists investigations against Ōshika Construction and its political collaborators by providing insider knowledge of the company's operations and corrupt practices, serving as a direct link due to his former position. Within the team, he functions as a source of comic relief, frequently displaying anxiety, nervousness, and a fearful demeanor that contrasts sharply with Zaizen's confidence and assertiveness, highlighting his role as an ordinary civilian thrust into high-stakes scenarios.

Throughout the series, Takumi adapts to GCIA operations while retaining his apprehensive personality. He contributes by leveraging his familiarity with Ōshika's internal structure and personnel. His presence underscores the human impact of corruption, representing individuals vulnerable when challenging powerful entities. He shows no significant evolution into a more assertive or combat-oriented role; his value remains his contextual knowledge and his function as a grounding, relatable figure amidst political and criminal intrigue.