"Ryoko's Case File" is a supernatural mystery anime centered around Ryoko Yakushiji, a brilliant and assertive investigator working for the Metropolitan Police Department in Tokyo. Known for her sharp intellect, unorthodox methods, and striking beauty, Ryoko tackles a series of bizarre and otherworldly cases that defy conventional explanation. Her investigations often intersect with ancient myths, occult phenomena, and advanced technology, blending science fiction with elements of horror and folklore.

The narrative unfolds as Ryoko, alongside her loyal and resourceful assistant Junichiro Izumida, delves into mysteries that challenge the boundaries of reality. From encounters with vengeful spirits to uncovering secret government projects, each case reveals a deeper layer of intrigue. The series frequently explores themes of power, morality, and the clash between tradition and modernity, as Ryoko navigates a world where the supernatural and the scientific coexist.

Key characters include Ryoko herself, whose confidence and determination drive the story forward, and Junichiro, whose grounded perspective and technical expertise complement Ryoko's bold approach. The series also features recurring figures such as Ryoko's rival, Yukiko Muromachi, a fellow investigator with a more cautious and methodical style, and Tatsuya, a mysterious figure whose motives remain ambiguous.

The plot is episodic, with each case presenting a self-contained mystery while subtly advancing an overarching narrative involving shadowy organizations and ancient secrets. The interplay between Ryoko's personal ambitions and her duty to protect the public adds depth to her character, as she balances her desire for recognition with her commitment to justice. The series maintains a steady pace, blending action, suspense, and moments of humor, creating a dynamic and engaging viewing experience.