Criminal Minds: Season 2, Episode 23

2 - 23: No Way Out, Part II: The Evolution of Frank

When the prolific and psychopathic serial killer Frank last seen in "No Way Out", returns and murders Gideon's girlfriend and old school friend, Sarah Jacobs in his own apartment, Gideon's own life is torn apart and he starts his own investigation before disappearing altogether. The BAU is forced to circumvent the local authorities to help Gideon find Frank, as a conflict of interest means that there's no way they're going to be asked for help on the case. Meanwhile, Strauss asks someone to give her inside information on Hotch and the way he runs the team. It's soon revealed that the mole she's chosen is none other than the BAU's newest recruit, Emily Prentiss.