When an FBI agent is brutally murdered in a gas station restroom the CSI team is called in to work with the victim's partners in order to find the violent killer. The FBI agents were working undercover to try and break-up an underage prostitution ring which appears to have contributed to his murder. As CSIs piece together the crime scene evidence they find an ultimate fighting champion hired by the leader of the prostitution ring was the likely killer. When confronted by officers, the suspect kills himself in front of hundreds of spectators. The team believes the case has come to a close as FBI headquarters gives them startling information; the victim was not an FBI agent and neither are his partners, who worked closely with Langston and Brass on the investigation; they are actually mental patients. When another one of them is murdered, CSIs know they must find the last "agent" before the ring leader does. Langston is the only one that can reach the surviving "agent" after he has a mental breakdown. With his help, the team takes down the leader of the prostitution ring and saves dozens of underage girls.