Yataro Sarumaru, a high school student nicknamed Saru, spends his days working at his father's locksmith shop in the Asakusa district of Tokyo. Though he is a genius at picking any lock, his skills with women are almost nonexistent, and he is constantly preoccupied with fantasies of meeting girls. Alongside his equally hapless best friend and co-worker, Kenji Yamamoto, Saru uses his extraordinary lock-picking talent to take on dangerous and mysterious cases that come his way, often hoping to impress a beautiful client.

The 13-episode live-action television series follows Saru as he navigates a series of episodic adventures. Each case typically involves a client in trouble, and Saru's ability to bypass any security system becomes the key to solving the problem, whether it involves a simple apartment door or a high-tech safe. His personal life is further complicated by the presence of his childhood friends: Ritsuko Mizuhara, an honor student who frequently scolds him for his behavior, and Jiro Yamada, a police officer with a strong sense of justice who often finds himself caught between his duties and his loyalty to Saru.

While Saru demonstrates brilliance and confidence when faced with a mechanical challenge, his immaturity and desperation for romance frequently lead him into comedic trouble. The narrative arc of the series focuses on his growth, not just as a locksmith, but as a person learning to navigate complex relationships and his own shortcomings. The dramatic and comedic tension arises from the contrast between his professional genius and his personal ineptitude, culminating in situations where he must rely on his unique skill not for personal gain, but to help his friends and do the right thing. The series concludes after resolving its central character dynamics and setting the stage for a subsequent feature film.