Christine Dale turns up news that an American assassin has been sent by the Iraqis to kill a high level British government official. Despite the fact that the CIA doesn't want MI5 to know of the assassin, Dale informs Tom Quinn, but he must keep it quiet.
Quinn investigates on his own, though unofficially informs Danny and Zoe, as he needs their help. Danny and Zoe are suspicious, especially since Tom doesn’t want Harry informed of their mission. Evidence starts to turn up that Tom might have turned traitor and the hunt is on.