Yoshitarō Katayama serves in the Tokyo Metropolitan Police's First Investigative Division, upholding a family rule established by his grandfather and continued by his late father mandating one detective per generation. He resides with his sister Harumi and their calico cat, Holmes, who possesses a sixth sense for solving mysteries. Professionally, Yoshitarō contends with significant phobias—blood, heights, ghosts, and women—often causing him to faint during investigations and leading to daily resignation letters. Despite these challenges, he capably manages household chores like laundry, cooking, and cleaning.

His personality is marked by a deep-seated belief in his unsuitability for detective work, frequently stating he must "not make a nuisance of himself." This insecurity arises from his inherent desire to trust everyone, particularly women, resulting in frequent misjudgments of female suspects. Holmes frequently provides crucial hints during investigations, uncovering solutions to locked-room murders or identifying culprits in seemingly inexplicable crimes. Yoshitarō alone sees Holmes take human form, facilitating their communication.

Family dynamics are central. His older brother Hiroshi manages the family's rental properties, acting as a paternal figure offering harsh but motivational advice; Hiroshi's obsession with mysteries aids their investigative discussions. Yoshitarō's younger sister, Harumi, is assertive and sociable, often involving herself in cases and providing emotional support. Their colleague Ishizu, who harbors romantic feelings for Harumi, alternately mocks and manipulates Yoshitarō, reflecting tensions between personal ambitions and familial loyalty.

Across investigations, Yoshitarō gradually develops confidence through Holmes' interventions and familial backing. For instance, he correctly trusts a male suspect in a final case, contrasting earlier misjudgments of women, indicating subtle growth in his deductive judgment. Despite solving complex cases, he remains self-deprecating, viewing his successes as flukes rather than earned achievements. His relationship with Holmes evolves from dependence toward partnership, though he continues to attribute breakthroughs to the cat's instincts rather than his own contributions.