After the prolonged build-up it’s good that the serial is now delivering. This episode opens with some of director Douglas Camfield’s trademark military action, with UNIT confronting Cybermen in the sewers with grenades. And when the cowardly Perkins (channelling the spirit of Driver Evans) gets gunned down, it’s notable that there’s both an immediate effort to retrieve his body, and a later scene where Isobel expresses her regret at his death. This feels a bit more true than all those later UNIT stories where the troops are redshirt cannon fodder for the monster of the month.

I think this is what people mostly mean when they talk about how “gritty” the earlier UNIT serials are: it’s all slightly more grounded and realistic when the action is taking place on the streets of London and deaths seem to hurt, rather than in the grounds of a country estate where the monsters vaporise humans in a puff of smoke and there are no unpleasant remains to clean up.

The script definitely has a hard edge: the photographs Isobel risked the Brigadier’s men’s lives for are dismissed as looking like fakes. The scene where Vaugn confronts Professor Watkins and goads him into an assassination attempt (which is laughed off) is brilliant. Later, Professor Watkins is rescued by UNIT off screen, but Gregory’s shell-shocked description of it makes it all sound very dramatic.

Poor Gregory. A couple of days ago the boss ruined his day by giving him an insoluble problem, and now he has to pay the price for Packer’s men’s failings. This results in the only bit that feels botched: one moment Gregory’s in Vaughn’s office, being grilled. A second later, he’s running from Cybermen in the sewers and is gunned down. Either the script was amended after location filming, or it’s a stylistic choice that doesn’t work.

The climax of a pretty tremendous episode is one of the all-time greats. Vaughn and Packer sit out the night waiting for the invasion to begin: Vaughn, with keen anticipation; Packer, with increasing doubts. Elsewhere in London, Isobel and Captain Turner watch the sun rise and wonder whether the Doctor’s story about an alien invasion can be true – just as it begins.

Then we get the money shots: so good they were lifted almost exactly to launch Doctor Who in colour in about 13 months’ time. Shocked Londoners react in horror as the Cyberman hypnotic signal begins to broadcast. Monsters burst out across the city, striding relentlessly past its landmarks. It’s The Dalek Invasion of Earth, but right here, right now. Iconic.

Next episode: “The Invasion” – Episode Seven