After a man dressed in a clown costume kills a bakery owner, the evidence is soon connected to a man from Flack's past who reluctantly agreed to join witness protection to testify against drug dealers.
An exclusive party in the back of an 18-wheeler ends in death when the driver sends the vehicle into the Hudson River, and the team tries to figure out what happened. However, a mobster whose daughter was killed may beat CSI to the killer.
A dead body is found in the remains of a collapsed porch from a high school party, and the team discovers that she was strangled before the accident. Meanwhile, a serial rapist from one of Jo's past cases strikes in New York City.
The team tries to identify the shooter and motive in the assassination of a criminal court judge presiding over the trial of a Russian mobster, while Mac and Lindsay prepare for the hearing against serial rapist John Curtis.