When construction workers find the body of an archbishop whose kneecaps were removed, the team traces the murder back to the case of the Widow's Son serial killer. Angela finds a victim tree in the evidence vault, which leads the team to believe the serial killer has a systematic pattern in selecting its victims. With this knowledge, the team tries to predict who will be the next victim and how they can stop the killer before it's too late. Meanwhile, Brennan's brother Russ comes out of hiding when he learns his girlfriend's daughter is sick, but his reappearance forces Booth to intervene.