When the bodies of two females are found in the desert outside of Las Vegas, the BAU is surprised to learn drowning is the cause of death. After the discovery of another victim, Reid notes the same cryptic message left at the two crime scenes is a hobo code used as a threat against stealing. The M.E. report reveals the victims' lungs were filled with water contaminated by ground pollutants which leads our team to the underground flood control tunnels beneath the city.They uncover a large homeless community inhabiting the tunnels and discover that all three victims lived there. The team quickly hones in on a delusional ex-con named Cesar who claims to be acting upon orders from "The Doctor". Reid is able to decipher Cesar's thought process to determine the Doctor is Marvin Caul, a low-level entertainer out to exploit the homeless population for financial gain. The homeless paid him to stay in the tunnels, and he manipulated some of the dwellers to kill those who stole from him or left his network. Marvin's plan was to garner enough money to book a stage at the Rio. The team tracks him down just as the brother of one of Marvin's victims is about to take revenge on him.