Episode 7

The siege at the Saudi Trade Centre moves to its terrible second act. Ros is inside, held hostage, while outside the team works the crisis with the clock running. The terrorists' demand is blunt: release the Al-Qaeda prisoners held in Saudi Arabia. To prove they mean it, they begin executing hostages, one by one. It aired in October 2006, a week after the attack that set it up.

The episode's twist is quieter than the violence. The attackers may not be Al-Qaeda at all. Suspicion creeps into every briefing, and the team has to negotiate with people whose true identity nobody can pin down. It is a hostage situation where the negotiators cannot even be sure who is holding the room, and the uncertainty changes everything about how they respond. The playbook for sieges assumes a certain kind of enemy; this one does not fit the file.

There is something properly unnerving about the format. The executions are not lingered on, but their rhythm is, and the show trusts the audience to feel the weight of each one. Hermione Norris's Ros, caught in the middle of it, gives the episode its human centre, and the scenes inside the centre are the reason it works.

David Wolstencroft and Raymond Khoury wrote it, Andy Hay directed. Peter Firth's Harry, Rupert Penry-Jones's Adam, Raza Jaffrey's Zaf, Miranda Raison's Jo Portman, Hugh Simon's Malcolm Wynn-Jones, Dhafer L'Abidine's Sharaf, Simon Kassianides's Mourad, Matt Day's Neil Sternin and Saskia Reeves's Sally Bernard work the crisis from every angle.

But the question the hour keeps returning to is who, exactly, the team is fighting. That doubt, more than the guns, is what makes the siege so uncomfortable, and it is one of the crueller episodes of the run. It keeps its nerve, which is the least the subject deserves.