Erin is prosecuting a murderous criminal. When her key witness breaks down when he was suppose to testify, Erin calms him down and schedules him to testify the following day, but he is killed, which means her whole case is dead. She speaks to someone who was there who claims to know nothing but Erin gets her to admit what she knows so she puts her in protective custody and Danny and Jackie are among the cops assigned to protect her. When they need something fixed they send for a repairman and when he arrives Danny senses something wrong about him and deduces he's a hit man sent to take out the witness. Which means someone in the team assigned to her protection told the criminal. So he takes the witness and puts her somewhere so only he and Jackie know where she is. While Danny and the Sarge try to find out who the leak is, Erin is being courted by the mayor to be his new deputy mayor which means among her duties will be looking over Frank's shoulder.