If you haven’t got the Nazi parallels by now, this episode makes them blatant: the extermination of all humans is ‘the final solution’. But despite the build up, the defeat of the Daleks is accomplished remarkably easily. Jacqueline Hill gets the funniest moments, firstly when she riffs on her history lessons to spin a yarn about an imminent rebellion, and then when she tries to imitate a Dalek voice over the radio, until the Doctor steps in with a more convincing impersonation. Following his orders, the Robomen turn on the Daleks and presumably kill every one they can find (shades of that in the future elimination of the Silents), while Ian sabotages their bomb causing the base to explode. Which means in both their appearances the Daleks are most disappointing in the final battle.

It’s a shame, because Richard Martin does make efforts elsewhere to add some pizazz to the direction. The Dutch tilt when the Black Dalek is issuing its extermination orders, and the moment when the Doctor, framed in a Dalek’s viewfinder, stares it down defiantly, is so iconic it’s been repeated endlessly in the modern series.

That said, the episode is less concerned with the Daleks than it is in writing out Susan. The Daleks have been safely dispatched by the 15-minute mark and the last third of the episode is all about the aftermath, making it the most developed departure scene possibly until Jo goes, certainly until Rose does. It’s interesting because Nation, or more likely Whitaker, contrasts the Doctor and David – the Doctor wishes Tyler luck in rebuilding the Earth, but has no interest in sticking around to see it. David wants to ‘see things grow again. I want to be a part of it.’ And the Doctor recognises that Susan wants that same security, and to belong in one place and time.

The method of exiting Susan – having the Doctor lock her out of the TARDIS with a very final-sounding slam of the doors (a scene played out in a kind of reverse in The Parting of the Ways when the Doctor locks Rose in) – wouldn’t wash today. The Doctor and David treat Susan as someone to be ‘taken in hand’. Overlooking that, it’s beautifully played by Hartnell. The little catch in his voice as he says his goodbye is as touching as anything in the next 50-plus years.

The Dalek Invasion of Earth as a whole is iconic: the Daleks in front of London landmarks has been repeated ad infinitum, both with them and various other monsters. But beyond that, it reconfigures the Daleks into the Doctor’s arch enemies, engaged in an implacable struggle with him across time and space. This also means the Doctor is reinvented as something more than an old man wandering aimlessly through time and space. He’s now the thing that gives the monsters nightmares. The seeds of Troughton’s ‘corners of the universe’ speech; of Dicks’ positioning of the Doctor as someone who couldn’t abide by the Time Lords’ non-interference policies; of Cartmel’s view of the Doctor as Time’s Champion; Davies’ ‘lonely god’ and Moffat’s legendary hero are sowed by Nation and Whitaker in these six weeks in 1964. And by cutting the cord with Susan, the person who’s been holding the Doctor to some kind of domestic responsibility, they finally allow him to become the hero the last 50 episodes have set up for.

Next episode: The Powerful Enemy