Episode 4

The job comes in two halves, and Adam Carter is expected to carry both. He and Zaf Younis fly to Istanbul to talk a young man away from extremism before the extremists get any more use out of him. At the same time Adam climbs into a truck packed with illegal immigrants moving across Europe, riding with them to stop a would-be terrorist before he reaches wherever he is going. Two threads of one operation, and neither leaves much room for error.

Back in London the team holds the picture together. Harry and Juliet Shaw watch the wider board, Fiona is never far from the action, and Ruth and Colin keep the machines honest. Khalid Abdalla's Yazdi is on the ground with Adam and Zaf, which makes the pair feel less alone than they probably are. Silas Carson's Prince drifts through the edges of the story, the sort of presence who arrives without an introduction and leaves without an explanation. It is the show's habit to send its people somewhere else for a week and let the distance do the work.

What lingers is the method. Most weeks the answer to a threat is a gun in a dark room. Here the mission is to talk someone out of a bomb vest, and the show is rarely more nervous than when its own people are trying to be reasonable. De-escalation is quiet, slow work, and it is hard to make thrilling. The episode manages it, which is rather the point.

Written by Howard Brenton and David Wolstencroft, directed by Alrick Riley, it gathers Peter Firth's Harry, Olga Sosnovska's Fiona, Anna Chancellor's Juliet, Nicola Walker's Ruth, Charles Daish's Phil Norton and Rory MacGregor's Colin Wells around Penry-Jones and Jaffrey. It aired in September 2005, and it has the feel of a season checking that its team can still work together away from home.

And yet it is a quiet kind of hour. Istanbul is a long way from the Grid, the stakes are personal as much as national, and the best the team can do is nudge one man in the right direction and hope he stays there. It is not the show's loudest episode. It may be one of its more honest ones.