Blue Bloods - Brothers

I had the pleasure of watching two episodes of Blue Bloods back to back.  This episode starts with a potential witness on the run from some goons but then he gets hit by a car, driven by the king of the street crims and the one he is to testify against.  An effective method for silencing witnesses.
Erin, the DA, played by Bridget Moynahan wants to get this guy but all the witnesses won't talk.  Her last resort is to convince his law-abiding brother to turn against him.  He isn't scared of his brother and says he will talk to him first.  What happens next is indicative of the brilliant writing of this show.  Nothing rarely happens as you think it would happen and no one rarely acts the way you think they will, keeping the audience on their toes and their eyes and ears glued to the television sets.
In parallel to this story is Jamie finding out more about what his deceased brother was investigating before his 'murder' and Danny riding him hard to make sure he is protected.  He can't lose another brother.
I have to ask though, what happened to Danny's partner?  Suddenly from one episode to another, he's got someone new.  What's with that?
Excellent episode, yet again.