MI5 begin investigating CCTV footage of a London terminal, where radioactive material has been detected and they discover that the handler of the goods was Johnny Grier – a man found to have been involved in illegal activity before.
Looking into the evidence, MI5 predict that they’re anticipating a dirty bomb, and that Johnny’s sister – Natalie – may be their only way in to finding out exactly what – and when – something is going to happen.
Dimitri is assigned to investigate, and Harry tells him to get as close to Natalie as he can. Earning her trust, he slowly gets to know both her and Johnnie.
Confronting Johnnie, Dimitri blows his cover and asks him what he’s planning to do. Johnnie tells him that he was smuggling some drugs into the country, and that he knows nothing about any radioactive materials.
Believing Johnny, Dimitri says that he will help him – and that he’ll get in touch with those who he believes are plotting. Tipping them off, Johnnie tells them his information. But it was a bluff, and Johnnie is already on his way to carry out his plot with the radioactive material. He ends up releasing it – killing himself, but not harming any others.
Meanwhile, Ruth edges closer to discovering who’s been impersonating Harry, and they also discover just who has stolen the laptop from MI5. It was the CIA.