Muldoon is tasked by his Aunt Kay to find the person behind his uncle, Mr. O's murder. Mr. O's son, Sean O'Bannon, also a Public Morals officer, starts a romance with best friend Duffy's sister, Deirdre. Shea gets a taste of the fun side of working in Plainclothes when Bullman takes him to a private, illegal casino.

Mr. O's wake brings together Muldoon and the west side Irish Mob, where Muldoon and Patton agree to meet and discuss the case. Also, Muldoon gets a tip about a high stakes, illegal card game run by the Italian Mob.

Muldoon and Latucci take their wives on a double date, using the perks of the job to have a night on the town. Shea is partnered for the night with O'Bannon, who has some fun at Shea's expense when arresting a couple of hookers. Kane takes the first step in avenging Mr. O's death.

Rusty takes revenge after making a major discovery. Muldoon steps in when Latucci's friendship with a well-known Italian Mobster is questioned. Bullman's relationship with high-end call girl Fortune is further complicated. Smitty quickly discovers that being a boss brings unexpected danger.

While looking for a killer, Muldoon and Bullman find themselves in the line of fire. Sgt. Mike invites Patton out to visit him to gain traction with the Mr. O investigation. O'Bannon is put to the test when Muldoon includes him in a pursuit.

Muldoon juggles tensions at home with his family, warns Latucci about his relationship with the Moose, while also pursuing the Pattons. Lt. King helps out an old flame and assembles some fellow officers to take care of a situation. Kane and Hopkins have been hiding upstate and plot their revenge. Patton questions his brother's loyalty as tensions rise.

Muldoon must make a final decision about whether Shea should be in Public Morals and puts him to the test. Kane finally returns to Hell's Kitchen, and it is soon a race between Rusty and Kane to get to each other first.

Muldoon gets a surprising tip. Patton is officially done with Rusty and starts making moves. Christine makes plans to leave Hell's Kitchen, with or without Muldoon. Deirdre gets unexpected news that forces her to look at her relationship with O'Bannon.

The streets of Hell's Kitchen run red with the blood of the Irish Mafia, as the west side war comes to a head. Muldoon treats Christine to a night on the town, and makes a decision. Duffy gets surprising news from Deirdre. As Bullman's feelings for Fortune intensify, he's forced to make a decision about their relationship.