Lu Yan, captain of the criminal investigation team, was once the youngest “crime-solving legend” in the police force. But three years ago, he wrongly identified a suspect in an unsolved case, allowing the real killer to go free. The victim's family, driven to despair, died by suicide. After that, Lu Yan lost his drive and was transferred to the marginalized “Cold Case Unit.” Located in the police station basement, the unit holds twelve unsolved cases spanning a decade, each stamped “No Solution.”
After the new chief takes office, the Cold Case Unit is ordered to reopen the investigations before a deadline. While organizing the files, Lu Yan discovers that the twelve cases seem entirely unrelated: an artist hanged in a locked room, a high school girl who vanished on a rainy night, a businessman crushed by a chandelier, an unidentified body dismembered on the railway tracks... Yet their details conceal a disturbing pattern. A special bronze bookmark was left at every crime scene, each engraved on the back with a different Roman numeral, from I to XII. And each victim was clutching an “object” with no connection to their profession.
As the investigation deepens, Lu Yan discovers that an unseen hand seems to be controlling the cases. Even the unsolved case he mishandled years ago is intricately connected to the twelve. The “objects” in the victims' hands point to a medical experiment from ten years ago, while the numbers on the bronze bookmarks correspond to the experiment subjects' identification numbers. And he himself has long been caught up in a revenge plot spanning a decade. When the twelfth bookmark appears, Lu Yan finally understands that these twelve cases were never separate crimes. They are a carefully designed “trial,” and he is both the investigator and the one on trial.
