Blue Bloods Season 13 Episode 21 revolves around Frank's latest tiff with the mayor.

When Mayor Chase announces a joint initiative that he didn't consult Frank about, Frank takes exception. He holds his own press conference, where he says the mayor's idea about arresting homeless people and removing them from the streets is not viable and that he is as upset as everyone else by this news.

Garrett is annoyed that Frank is making an enemy out of the mayor and storms off to take some vacation days. Meanwhile, Frank tries to talk to Chase, but that only escalates the situation when a homeless guy attacks the motorcade and Chase accues Frank of engineering the incident. Later, Frank finds out some cops on the security detail did it, but won't tell Chase who they are. Chase tells Frank to join him on the podium tomorrow or he'll go to the press.

Elsewhere, Erin gets frustrated with Jamie and Danny when they both complain to her about perps who got released and harmed new victims. Danny's case involves a serial killer he thought he already put away, and it's in Jackie's neck of the woods so she suggests they work the case together.

To find out what happens, watch Blue Bloods Season 13 Episode 21 online.