After construction workers find the body of an archbishop whose kneecaps were surgically removed, the team is back on the case of the Widow's Son serial killer. When Angela discovers what appears to be a victim tree in the evidence vault, the team realizes the killer employs a systematic pattern in selecting victims. With a musician and an archbishop the first to go, the team tries to predict who might be next and how they can stop the killer before he strikes again. Brennan and Booth continue to see Dr. Lance Sweets, who has some helpful psychological insights into the Widow's Son's motives. Meanwhile, Brennan's brother, Russ, comes out of hiding when he finds out that his girlfriend's daughter is extremely sick, but his appearance causes Booth to intervene with Brennan's family once again.