The BAU is called in when the bodies of two drowned women are found in the desert outside Las Vegas. Painted on a sign near one of the bodies a mysterious note: "2/10".
Meanwhile, a drifter (Finn Bailey) wanders into a Las Vegas restaurant, looking for a job. An old man (Marvin Caul) catches Finn attempting to lift a wallet. Marvin offers him to help him hone his pick-pocket skills, as well as a place to stay.
Upon reaching the underground homeless city, they encounter a man named Caesar, who acts as an enforcer for someone they call "The Doctor". A little later we see Caesar deciding to kill one of the con artists for not paying his due "taxes" - the percentage of each person's "take" for the day.
Later, police find the body, again with the note "2/10". Reid and Blake realize that it's a symbol to indicate "thief' and is being painted as a warning to other thieves.
A woman shows up at the underground city, pretending to be in need of a place to stay until she can get a bus ticket to get back home. When Caesar challenges her, Finn vouches for her.
A little later Caesar discovers she's a reporter and he knocks her unconscious in preparation to killing her - all under the instructions from "The Doctor". Before he can kill her, Finn shows up to stop him. As Caesar and Finn begin to argue, Marvin intervenes and suggests that since Finn vouched for her, he should be the one to kill her.
They take her (still unconscious) outside and Finn takes out a gun to shoot her. Instead, he turns the gun on Marvin, accusing him of killing his sister. Finn's reason for being in Las Vegas was to find his sister, who had previously written to him about "The Doctor" - now identified as Marvin.
Before he can kill Marvin, the BAU show up. In order to de-escalate the situation, Reid lies to Finn, telling him his sister died in a crack den in a different city. He then lies to Marvin and manages to get a confess to killing Finn's sister and burying her.