Ep 7
Entrapment
So another "filler". The gist we get here, is humans and mermaids have lived in harmony for a very long time. Then the Pownall incident happened.
It was Sean's body they discovered, and as expected, Xander, Calvin, Ben and even Chris are all arrested. Lt. Martinez confronts Ryn, and in exchange for her willingly going with the military, all charges are dropped. So the guys go free. However they figure out they must have a spy among them. Xander just figured out who it is, but as usual says nothing.
So what happens when the whole gang gets back together and discovers what Ryn did? Should be obvious.