Season 8, Episode 5:

Section D is caught on the back foot when two CIA agents are found dead. As the team tries to make sense of the murders, more officers are killed. MI5 is placed on high alert, but Ros insists on putting herself in the firing line in order to draw out the killer. Will the risk pay off, or will Ros become another victim? Meanwhile, Ros's mentor raises questions about Harry's past, and Ruth makes a shocking discovery...