Internal Affairs

An agent who was undercover with a drug cartel went missing, abducted by a serial killer hiding in the violent culture of Mexican drug traffickers.

Tropes in this episode

Callback: Hotch is extremely cautious about approaching his car after an informant was killed in a car explosion, probably because he almost was a while ago, too.
Mole in Charge: It turns out that the boss of the boss of the boss of the DEA agents is the mole.
Red Herring: The head of the NSA suspects the local DEA chief of being the mole, and he seems suspicious enough, right up until he is murdered.