Danny contemplates retiring until Erin enlists him to help her with a case involving her ex-husband, Jack. Also, Jamie and Eddie go undercover as a couple, and Frank butts heads with the new mayor of New York City, Margaret Dutton.

Erin's trial against a notorious human trafficker is jeopardized when the presiding judge is mysteriously found dead. Danny and Baez are put on the case to investigate foul play. Meanwhile, Mayor Dutton opposes Frank's public safety initiative, but City Council Speaker Regina Thomas devises a plan to help Frank bypass the mayor's veto.