For three years, a serial killer has walked among the citizens of Hangzhou, China. With each new body come the same clues: a jump rope, a cigarette, and a slip of paper that says, "Come and get me."

In the same city, Luo Wen wanders into a park just as a young woman stabs a local thug to death in a desperate act of self-defense. A former criminal investigator who lost his own family in an unsolved case, Luo Wen is familiar with the slippery nature of justice. So he makes the split-second decision to help the terrified woman cover up her crime, leaving behind no trace of evidence.

Luo Wen's friend Yan Liang, a criminologist and professor known for solving each case like a mathematical equation, is called in to help the police on the recent homicides. As he digs deeper, he discovers more questions than answers--and variables that simply don't add up.

While Yan Liang follows his suspicion and Luo Wen covers his tracks, a battle of wits plays out--with the winner's version of justice as the ultimate prize.