"Onihei" is a historical anime series set in Edo-period Japan, centering on Heizo Hasegawa, the chief of the Arson Theft Control, a specialized police force tasked with combating crime in the bustling city of Edo. Known as "Onihei" or "Demon Heizo" due to his fierce reputation and unyielding sense of justice, Heizo is a complex figure who balances strict enforcement of the law with a deep understanding of human nature. His approach often involves not only apprehending criminals but also seeking to reform them, offering second chances to those who show remorse or potential for change.

The series is episodic, with each episode presenting a self-contained story that explores the lives of various criminals, their motivations, and the societal conditions that drive them to crime. Heizo and his team, including his loyal subordinates like the skilled swordsman Omasa and the resourceful informant Tetsunosuke, confront a wide range of adversaries, from petty thieves to masterminds of elaborate schemes. The narrative delves into themes of morality, redemption, and the gray areas between right and wrong, often portraying criminals as multifaceted individuals shaped by their circumstances.

Heizo's personal life is also woven into the series, providing insight into his character. His relationship with his adopted daughter, Omitsu, and his late wife, who was killed by a criminal, adds emotional depth to his relentless pursuit of justice. The series frequently juxtaposes Heizo's stern exterior with moments of compassion, highlighting his belief in the potential for human change.

The setting of Edo is richly depicted, capturing the vibrancy and challenges of the era, from the bustling streets to the shadowy underworld. The series maintains a grounded tone, avoiding exaggerated action in favor of realistic portrayals of samurai combat and investigative work. "Onihei" stands out for its focus on character-driven storytelling and its exploration of the moral complexities inherent in law enforcement and criminal behavior.