When an assistant U.S. attorney overdoses and ends up left for dead, Decourcy Ward suspects Jackie Rohr's involvement. Jackie, in turn, goes after the BPD for a questionable drug arrest at Braxton Summit, a federal housing project. Community activist Grace Campbell recruits Siobhan Quays to work on behalf of Braxton and its residents, but their efforts are subverted as tensions flare between rival gangs.

As Decourcy increases pressure on the Justice Department, Jackie counters by looking into his adversary’s romantic past. While Jackie’s digging up dirt, Siobhan is planning ahead, focused on starting a family even as Decourcy remains preoccupied by the escalating conflict. Jenny realizes she can’t keep ignoring her mother after an unsettling phone call. With business at the salon all but dead, Cathy Ryan turns to an unlikely income source.

Jackie uncovers one of Decourcy’s informants and hatches a plan to use him as a pawn to discredit the assistant district attorney. Decourcy stops by the Rohr house, hoping to get something out of Jenny that he can use against her husband. Siobhan receives some welcome news, in the midst of plotting her run for City Council. And her client, Kelvin Campbell, is released from custody, though he can’t free himself of what happened on Valentine’s Day. Jimmy Ryan, meanwhile, faces problems far worse than a guilty conscience.

After seeing Holly in the ICU, Jackie reckons with the destructive consequences of his actions, as Rickey Townsend prepares to testify against Decourcy. With violence intensifying between the Copeland Crew and the Braxton Boys, Decourcy, Anton and Siobhan face off in the interrogation room. Grace, left reflecting on her role as a mother and leader in the community, makes an unfortunate miscalculation. And back at the Rohr house, Jenny makes fast friends with the mysterious woman from Belfast.

As U.S. Attorney Karen Shimizu tries to get rid of her troublesome agent once and for all, Jackie schemes his way into a new mission. At the Suffolk County Courthouse, Decourcy and Siobhan spar on opposing sides of a legal battle. Meeting dead-ends at every turn in the effort to help her son, Grace does something desperate. With a nudge from Maeve, Jenny surprises Benny with some rebellious fun. On the other side of town, Cathy re-ups her supply by going straight to the source.

With Anton on the lam, Kelvin struggles to lead the Braxton Boys and Jackie homes in on Grace. He knows she's hiding something. Sidelined from the investigation, Decourcy finds himself at a disconnect with Siobhan, as he seeks vengeance and she focuses on recovery. When Cathy’s arrangement with Isaac falls through, she pays a reluctant visit to old friends for help. Jenny, meanwhile, finally uncovers the truth about her new friend.

Assuming the involvement of the Campbell brothers at the Copeland territory shootout, Jackie confronts Grace, who can no longer sidestep her sons’ criminal activity. She is forced to make a terrible compromise. Resenting the outcome of a negotiation that he could not take part in, Decourcy finds comfort in the strength of Siobhan, who keeps her head even after tragedy strikes. Jenny insists on helping Maeve, whether or not that means defying Jackie.

As Boston mourns another loss, Decourcy pursues justice in and out of the courtroom. Despite his maneuvering, Jackie discovers that he can’t escape his past, while Jenny decides to finally confront hers. Cathy says goodbye to the root of her struggles. Siobhan sets her sights on a new dream.

In early ‘90s Boston, racism is endemic and crime is widespread. Jackie Rohr, a renowned FBI agent whose methods are not exactly by the book, does not mind the status quo. But Decourcy Ward, the new Assistant District Attorney in town, wants to uproot the corrupt system. The two form an unlikely, and obligatory, alliance to take down a gang of armored truck robbers from Charlestown.

After an eventful Sunday at church, Decourcy finds out he has a new powerful enemy in town — one that puts his wife Siobhan in an awkward position. Jackie is stirring up trouble in his own home, and his distrustful mother-in-law Rosa is onto him. Investigators Hank Signa and Rachel Behnam look into an unsolved missing persons case connected to the robbery in Revere. That could mean trouble for Frankie Ryan, who’s busy laundering his most recent loot and, with the help of his brother Jimmy, getting carried away in the process.

Jackie searches for his absent informant as Jenny searches for purpose, revisiting her passion to become a teacher. Decourcy works up a plan to overcome the town’s infamous “Code of Silence” and get a few suspects to talk. Minogue is assigned a big case, but not the one he actually cares about. Jimmy skips town to visit his kids and finds that, even a thousand miles away, he’s not safe from his problems back home.

Jackie strong-arms Jimmy to get a step closer to finding the presumably dead armored truck guards. Decourcy rattles a suspect called to testify before the Grand Jury. Siobhan learns something unsettling about a leader of the Coalition and worries it could jeopardize the future of the group. Kick’s nightmares continue as she senses her father might be in harm’s way.

Jackie and Decourcy continue to hunt down the weapon connected to the armored truck robbery in Revere. Michaela Freda, a local reporter, gets a tip about a story that could lead her down a treacherous path. Despite the threat of a Grand Jury, Frankie considers what he must do to provide for his family. Pressure on Siobhan begins to mount. After another allegation is brought to her attention, she questions whether doing what is right is ultimately what is for the best.

Decourcy is a step closer to connecting the stolen guns to the Revere robbery when an insider agrees to talk. Jenny is no longer able to ignore Jackie’s indiscretions after they show up on her doorstep. An unexpected rush to the hospital worsens money problems for the Ryan family and Reverend Fields discovers that his longstanding influence doesn’t make him invincible.

With the gang’s arms dealer in custody, Jackie thinks he’s a step closer to glory. But Decourcy is the one in a position to negotiate and he’s considering cutting off the gun supply rather than going after the buyers. Jimmy and Frankie must find a new dealer as they prepare for their next job. Cathy and Jenny struggle to comfort their daughters who’ve undergone trauma. When Michaela falls into some trouble, Rachel decides to take her investigation into her own hands.

Jackie celebrates a win in front of the cameras but, behind closed doors, he reaches a new low. A shining moment from Decourcy promises a potential future in politics, but any pride Siobhan feels for her husband is squandered by an unexpected betrayal. The Grand Jury reconvenes, and this time Sheik doesn’t get off easily. Thanks to Signa’s detective work, the Ryan family is now under the watchful eye of the Suffolk DA.

When an unlikely source confides in Decourcy, the Massachusetts State Police hatch a plan that promises arrests without the help of the FBI. However, keeping out of the way is not exactly Jackie’s strong suit. The Genesis Coalition pitches a new strategy to curb Boston’s crime rate. Jenny opens up about her past to a sympathetic ear.

Jackie celebrates another moment in the spotlight, though an unfamiliar pang of guilt leaves him reflecting on his legacy. Decourcy considers the best course of action to finally carry out justice for the murdered guards. Jimmy is desperate to weasel his way out of the mess he’s created, even if it means putting his brother’s future into question. Jenny faces what has been haunting her all these years. Season finale