After getting a match from an unknown relative from a DNA network, Sean Reagan does some detective work to figure out who this mystery person might be. Frank gets a personal request from a woman to transfer her son to a safer assignment. Also, Danny seeks to uncover the truth behind the murder of a key witness in one of Erin's cases, and Eddie begins to think about having a child after she and Jamie discover a newborn left outside their precinct.

Amidst a shifting political climate, Frank goes head to head with City Council Speaker Regina over protests against police brutality. Also, Jamie and his nephew Joe Hill work together to locate Danny and Baez when they go missing while searching for a killer, and Eddie steps up to help an abrasive woman find her father's body after it was misplaced during the outset of the pandemic.

Shockwaves ripple through the Reagan family when Danny discovers that their newest family member, Joe Hill (Will Hochman), is working undercover for the ATF to bring down a gunrunning organization.