Castle catches up with Beckett at the office of Deon Carver, a bail bondsman who has been hit over the head. Whoever dialled 911 left the phone off the hook to alert the authorities. Lanie reckons that the call was made about an hour after Deon died, and she finds a partial shoe print in his blood.
Some feedback on her radio leads Beckett to discover a bug planted in Deon’s calculator. Deon’s wife Brooke says that while he had a dirty job, he was a man with a good heart. Called to Lanie’s autopsy room, Beckett and Castle are handed a piece of paper bearing a series of peculiar shapes that Lanie found in the victim’s sock.
Esposito gets a lead from a waitress who works in a diner near Carver’s office. She tells him that she heard an agitated guy say to Carver that if he did not give him something, he was going to take it. The suspect is identified as Random Pierce, who has been only recently released from jail.
A young mathematician is killed with a 200-year-old bullet.