Danny believes a serial killer is on the loose when two separate murder victims are identified as members of a group charged with a gang rape. Meanwhile, Erin’s ex-husband, attorney Jack Boyle, resurfaces in her life as they face off in court.

Danny’s car matches an NYPD description of a car involved in a drug deal and taken into custody when a bag of cocaine is found in his trunk.

Danny becomes the lead hostage negotiator when a prime informant in a large drug case holds Erin at gunpoint inside the courtroom.

Frank and Garrett devise a plan to convince Mayor Chase to work better with others in order to properly run the city, and Danny and Baez try to track down a perp who vandalized several NYPD vehicles with anti-cop graffiti and made threats against officers. Also, Erin is shaken after she's in a car accident, and Eddie and Jamie argue about whether they should keep a stray dog who leads them to a crime scene.

Business becomes personal for Frank and his 1 Police Plaza team when Baker is assaulted on the street, as well as for Erin, when she asks her ex-husband, Jack Boyle, to represent a defendant she’s prosecuting. Also, Eddie wonders if she’s being too stubborn after she publicly undermines one of Jamie’s decisions at work, and Danny and Baez investigate the attempted murder of a couple with huge secrets.

Eddie and Jamie deal with a domestic violence case associated with an investigation lead by Danny and Baez; Frank and his friend go on a mission to avail themselves to an unfiltered look at the city; Erin is presented with a surprising offer.

Danny crosses paths with a criminal from a previous case when he investigates a gang attack with a shocking motive; Frank and Mayor Chase butt heads over an off-duty cop who goes too far making a political statement.

Danny and Baez race against the clock to catch a serial killer targeting young women; Erin and Eddie clash when Eddie intervenes in a murder case and Erin's personal life; Frank must decide how to discipline his grandson.

Jamie learns that Eddie’s close friend Tracey may be using the restaurant she owns as a drug front; Erin is accused of causing the suicide of a former colleague; Danny and Baez are faced with various descriptions of the same suspect.

Danny and Baez investigate a case of grand larceny tied to feuding crime families; Frank clashes with Mayor Chase when an influx of immigrants arrive in the city; Erin questions whether her district attorney campaign is compromising her day job.

Danny and Baez team up with Danny's old partner to find a copycat killer emulating a criminal's murders from a previous case; Frank and Mayor Chase battle over how best to handle the city's rising homeless population.