The story takes place in Philadelphia, the United States. The central character is a homicide investigation team. The case they are facing is not a recent one, but an old case from several years ago or even more than ten years ago. Because it happened a long time ago, the parties involved are difficult to find, and all kinds of criminal evidence have almost disappeared, so it is particularly difficult to investigate. Such homicide cases are vividly called "cold cases". The protagonist of the series is Lily Rush, the only female detective in the "cold case" investigation team. With her tenacious character, she and her teammates search for the parties involved who are still alive, and unravel the past events that have been dusty for many years.