Mark Gregory has killed four women so far along Florida's Gulf coast. The BAU are called in on the case because Gregory has gone missing. A woman named Nicole Wegener is also missing and is assumed with Gregory as his next potential victim. The common link between the killings themselves is the ritual nature of drowning, which the BAU believe is Gregory's sexual release. If and when he kills again, they believe the drownings will become a more pronounced aspect of his process. They also believe that his killings will have a progression, that he is headed somewhere specific to get the ultimate satisfaction. The BAU want to check his past to see if it provides any indication of where he may be heading or who his final victim may be. In speaking to Mark's father, Doug Gregory, Gideon and Reid don't think he's telling them the entire story. But the BAU learn that it's not who his final victim will be but who his first victim was that is the key to finding him.
—Huggo