Episode 3

The Grid does not do guests, which is why having an FSB officer walk the corridors is a small crisis in itself. But the Russians are here to help hunt Azis Aibek, a rogue nationalist from the ex-Soviet state of Azakstan, who is in the UK for one thing: the last known sample of the nerve agent Paroxocybin. The team has to stop the stuff reaching the wrong hands without setting Anglo-Russian relations back a decade, which is the sort of assignment where every courtesy is a threat.

The chase drives the hour, and the uneasy alliance keeps it interesting. An FSB officer in the building changes the atmosphere of every room, a tension the show usually saves for interrogations. Off the clock, Lucas's personal life takes a passionate turn, which is this show's polite way of saying that nothing about it will end simply. The two halves sit oddly together, and the episode does not try to force them into one mood.

Richard McBrien wrote it, and Michael Caton-Jones directed. Peter Firth's Harry has to manage the diplomatic side of the operation while Richard Armitage's Lucas does the running. Nicola Walker's Ruth, Shazad Latif's Tariq, Sophia Myles's Beth and Max Brown's Dimitri Levendis are on hand, with Simon Russell Beale's Home Secretary, Iain Glen's Vaughan Edwards, Laila Rouass's Maya Lahan and Jonathan Aris's Aibek completing the picture. The FSB officer is a presence more than a person, which is the point.

But the show has done nerve agents before, and rarely with this much restraint. The threat is quiet and the guest is unreadable, and the episode trusts that to be enough. It earns its place by being professional, and it does not need to be spectacular. It is a holding-pattern episode, the kind that moves pieces into place without announcing it, and it does that job well.