Inspector Koichi Zenigata faces profound professional and emotional turmoil when ICPO leadership removes him from the Lupin III case. This demotion shatters his life's purpose, leading to uncharacteristic behavior: he confides the news of his reassignment over drinks with Lupin and Jigen. His identity in crisis, Zenigata reveals his new target is the international arms traffickers, Shot Shell.

Despite no longer being tasked with Lupin's capture, he willingly partners with the thief and his associates to dismantle Shot Shell, motivated by both a desire to regain his position and personal conviction. This cooperation requires significant flexibility from the inspector. During the mission, he actively participates in stealing a Russian nuclear submarine and kidnapping its necessary operator, scientist Karen Korinsky. His integration into the thief team deepens, extending to performing menial tasks like domestic chores aboard the sub.

Zenigata strongly disapproves of the lethal methods employed by his replacement on the Lupin case, Keith Hayden. He maintains his principle that Lupin must be arrested and face trial, not executed, despite his own temporary collaboration and personal grievances.

His emotional state remains volatile throughout, including moments of tearful sentimentality when expressing gratitude to Lupin for their joint effort against Shot Shell. Their dynamic reflects a complex mutual reliance, with Zenigata temporarily setting aside his duty to capture Lupin for the greater mission, even while intending to eventually arrest him.

Following the mission's conclusion – involving the submarine's destruction, the elimination of Shot Shell's leaders, and the exposure and death of the corrupt Keith Hayden – Zenigata immediately reverts to his primary objective. He attempts to arrest Lupin despite their recent partnership and Lupin's role in restoring his professional focus, reaching for his handcuffs in a final reinforcement of his defining pursuit.