A retired BAU man, "Max Ryan," who a lot like Jason Gideon, is giving a talk with the presentation of a his book about a serial killer from 20 years ago. The "Keystone Killer" murdered seven women in Philadelphia and was never caught. After the talk, the lecturer is given a letter. Apparently, the killer was sitting right in the room and apparently he's back killing again as one of the women he mentions in the note is found dead.
This was a very good episode as the tense ex-BAU is emotionally involved and the current team is not as they try to figure out who this guy is and why he stayed silent for 20 years before resuming his killing with a different MO.
Without spoiling anything, I have to say this killer sure didn't look like one!
On the personal side "Ryan," I presume was supposed to show what years on the BAU team can make you into; something with no life. no family...and way too serious. His behavior doesn't go unnoticed, especially by "Elle" who worries this will happen to her.