When a diamond heist results in an off-duty police officer being killed, Frank and the entire NYPD are determined to catch this cop killer. After a few leads, Danny and his new partner Jackie Curatola find their main suspect is Ricky Russo, the son of a famous old-school mob boss nicknamed "Happy Jack." Henry is surprised to learn of Happy Jack being involved as he was his advesary back in the day but believed him to be a "good" bad guy who would never associate himself with a cop killing. After Henry goes to see Happy Jack, the cops soon are able to track his son down and he confesses to them who pulled the trigger and is responsible for the cop's murder. Meanwhile, Jamie and Renzulli deal with a couple who make their domestic dispute a regular activity. Erin must deal with the difficulty of raising her teenage daughter who is back from vacationing with her father and acting out as a result.