Episode 6

Ros Myers goes undercover with the Saudi royal family, looking into a prince who may have ties to al-Qaeda, just as the family's official business brings them to London. A delegation of Saudi princes is offering Britain a guaranteed oil supply in exchange for nuclear expertise, which is the sort of deal that makes everyone at the table very polite and nobody trust anyone else.

Adam and Zaf, meanwhile, are hunting a group of terrorists who have already infiltrated British life, which gives the episode two hunts running at once. The Saudi Trade Centre attack, when it comes, turns the polite summit into an emergency, and the delegation's visit stops being diplomacy and starts being a security incident. The attack lands in the middle of the visit, which is either terrible luck or perfect timing, and MI5 has to assume the second.

The episode earns its tension from the collision. On one side, a royal visit so guarded it should be airtight; on the other, a cell that has already proved it can move through the country unseen. Hermione Norris's Ros carries the weight of the first, and Rupert Penry-Jones and Raza Jaffrey carry the second. Ros's cover puts her closer to the royal family than any outsider has a right to be, and every courtesy she receives is a clue as much as a kindness.

David Wolstencroft and Raymond Khoury wrote it, Andy Hay directed. Peter Firth's Harry, Gugu Mbatha-Raw's Jenny, James Dicker's Wesley Carter, Raad Rawi's Saudi Ambassador, Ian Bartholomew's Daniel Wise, Dhafer L'Abidine's Sharaf and Simon Kassianides's Mourad fill out the cast.

And the sharpest thing in the hour is the deal itself. Oil for nuclear know-how, signed with a smile, while the bombers are already inside the country. The episode does not have to reach for a villain. The guest list provides enough menace on its own.