Episode 6

The American president stops in London without warning, and suddenly MI5 and the CIA are working the same city, with neither side happy about it. The two organisations circle each other like dogs that have been introduced but not told to play nicely. Tom is thrown together with Christine, a CIA agent, and the episode wastes no time suggesting there's more between them than the work. His ex-girlfriend Miranda is less than delighted, and the question of whether she leaves well alone becomes its own subplot. Everyone is watching everyone else, and the watchers are being watched.

But the power struggle is the main event. Both sides want to be the one making the calls, and the mistakes start to look expensive before anyone has fired a shot. It's the kind of week where the biggest threat is the ally in the room, and the episode plays the tension well, letting the professional rivalry curdle into something personal. David Wolstencroft wrote it, Justin Chadwick directed. The episode's best scenes are the ones where the two sides are being friendly.

Peter Firth's Harry keeps MI5's end as composed as he can manage, Keeley Hawes's Zoe backs him up, and Megan Dodds's Christine gives Tom's scenes a charge that's more than professional. Ruth Gemmell's Miranda, Todd Boyce's Troy and Stephen Kirby's Trevor fill out the edges, with Tim Bentinck's RAF controller supplying the logistics.

The episode is a reminder that Spooks was never only about foreigners with bombs. Some of its best hours are about the allies you can't trust, and this is one of them. There's a particular tension in watching two teams that are supposed to be on the same side keep score, and the episode milks it for everything it's worth. The president moves on, but the two services carry the week's bad feeling with them.