Strickland's Whitehall "pal" Stephen Fisher is targeted by an assassin, so the UCOS boss enlists the team to find whoever was responsible. He reveals that 30 years earlier, he and Fisher were involved in a covert MI5 operation to break into the house of a journalist believed to have information on an IRA arms deals. But with several of the original agents having died in suspicious circumstances, he believes someone is out to kill them all - and when a connection is made between a gangster and MI5, it seems the threat may be coming from within the security service itself.