"RIN - Daughters of Mnemosyne" is a supernatural thriller anime set across multiple decades, beginning in 1990 and spanning into the distant future. The story revolves around Rin Asōgi, an immortal private investigator who runs the Asōgi Detective Agency in Tokyo. Rin, along with her assistant Mimi, takes on cases involving supernatural phenomena, often tied to the mysterious Time Fruit, a rare entity that grants immortality to those who consume it. Rin herself is an immortal, having consumed the Time Fruit centuries ago, and her immortality comes with the ability to regenerate from nearly any injury, though the process is excruciatingly painful.

The narrative unfolds non-linearly, with each episode jumping forward several years or decades, showcasing how Rin and Mimi navigate a world where immortals, known as "Aeon," are hunted by a secretive organization called Chronos. Chronos seeks to control or eliminate immortals, viewing them as threats to humanity. Rin’s immortality and her connection to the Time Fruit make her a prime target. Over the course of the series, Rin encounters various immortals, including Kouki Maeno, a man who becomes entangled in her life after a chance encounter. Kouki’s involvement deepens as he discovers his own connection to the Time Fruit and the immortals.

The series explores themes of immortality, identity, and the consequences of living beyond a natural lifespan. Rin’s relationships with other characters, particularly Mimi and Kouki, evolve over time, shaped by the passage of decades and the constant threat from Chronos. The story also delves into the origins of the Time Fruit and the enigmatic Mnemosyne, a being tied to the creation of immortals. As Rin uncovers more about her own past and the true nature of the Time Fruit, she becomes embroiled in a larger conflict that challenges her understanding of immortality and her place in the world.

The series is notable for its mature themes, graphic violence, and explicit content, which are integral to its exploration of the darker aspects of immortality. Rin’s journey is one of resilience and survival, as she confronts both external threats and the internal toll of living forever. The narrative’s structure, spanning decades, emphasizes the weight of time and the inevitability of change, even for those who cannot die.