The BAU investigated the case of a missing child, who later turns up dead. This is identical to a fifteen year old cold case. They discovered that the Unsub was a man who saw his Eastern European mother being tortured by his father when he was young.  His mother had tried to free herself and her son by committing murder-suicide by driving her car into a lake.  The boy escaped, but later on saw other families as surrogates - families who had the same characteristics as his own: man married to Eastern European woman, with a small child.  He murdered the children as a means of protecting them. He also saw the mother as a victim and decided to kidnap her as well, but the team found his house in time to rescue her from where he'd hidden her: a secret compartment behind an altar.  Blake had her back turned to a secret wall and didn't realize the Unsub was coming out of it to kill her.  Reid followed the secret passage way from another room and emerged in time to see the Unsub and shoot him.