Episode 6

Dean Mitchell is a teenage boy who stumbles on a government conspiracy and spends the whole hour paying for it. The stumble is the whole premise: a glimpse of something he was not meant to see, and suddenly he knows things that get people killed. He is barely inside MI5's world before he is on the run, with Lucas and the rest of Section D acting as bodyguards to a frightened kid while the people who want him silenced close in. But the oldest story in the espionage book is rarely told from the side of the innocent who wandered into it, and that is where this one earns its keep.

Harry Pearce is running a quieter version of the same anxiety. A cryptic message from his asset in Russia points at corruption high in Whitehall and, worse, at a mole buried inside MI5 itself. For most of the episode the boy's danger is the loud one. And the other threat hums underneath, waiting for its moment, the way the show's best fears always do.

Edward Hall directs David Farr's script. Peter Firth anchors it as Harry Pearce, with Richard Armitage's Lucas North leading the protection detail. Hermione Norris, Miranda Raison, Gemma Jones, Hugh Simon and Alex Lanipekun fill out the Grid. Jacob Anderson brings a genuine helplessness to Dean Mitchell, Shaban Arifi plays Dmitri Volyakov, and Jill Baker is Rachel Beauchamp.

Watching a unit that lives on secrets suddenly responsible for one ordinary life raises the temperature in a way ticking bombs rarely manage. The episode lets the boy's danger and Harry's dread run side by side, and neither gets a tidy answer. Dean is the reason to watch. The mole is why nobody relaxes, and the hour leaves both threads open, which is the honest way to end something this tangled.