Brennan, Booth, and Zack rappel down a ventilation shaft leading to a tunnel beneath Washington DC. When they reach bottom they meet with some police officers, who are guarding the remains of a human body. Brennan borrows Booth's gun so she can kill some nearby rats, giving the rats that are feasting on the body something else to eat. A preliminary examination of the body reveals that the victim is a twenty-something female. Brennan notices a man standing in the tunnels observing the group. When the man runs off, Brennan gives chase, but the man gets away...