The team discovers a sleeper agent with terrifying connections in the US military. Faced with a formidable foe who not only appears to be a traitor, but a trained spy with access to US bases, MI5 may be out of its league. Meanwhile, Daddy is on a mission of his own to secure the safe defection of his favourite Chinese dancer.

The team is reeling from the shocking discovery of a mole among them, and as the Soviet plot continues unabated, it is up to Wendy to give the team the break they need. Meanwhile, when Sarah finds herself in danger, Alan is forced to make a decision that can make or break their relationship. Joe has trouble of his own when leaked information threatens the investigation and the lives of the team.