This episode’s title card is black with white text – the inverse of the previous three episodes’. That’s either a deliberate reference to the growing darkness of the serial; the way the Doctor reverses the polarity and switches the War Machines’ power against WOTAN, or more likely just a mistake.

In every respect, this is one of the great end-of season episodes. So much of it feels familiar reviewed through the prism of the RTD era, where each year ended with TV news reporters warning the world in peril, and panicked members of the public cowering in their own homes as everything went to hell outside. That exact thing happens here – the news is heard in a pub, on a car radio, in a living room. For the first time, “we” are in danger.

That immediacy is reinforced by the vérité style direction – War Machines charge through the streets of London, terrorising passers by. An abandoned bicycle wheel spins. A man calls the police – and is gunned down as he does. Police cars warn people to remain indoors. There are scenes inside a busy newsroom as a reporter questions whether the threat is confined only to London – or whether it will go global. Best of all, Michael Ferguson has employed the actual ITN newsreader Kenneth Kendall to add to the sense of ‘reality’. This has a sense of scale and immediacy that has not been previously achieved in the show.

Against all these, for the first time Hartnell doesn’t look entirely convincing. He’s fantastic on film, boldly striding up to a captured War Machine to reprogram it. But he’s on less firm ground in the studio, where his charming habit of meandering around the point, getting across the gist of a line rather than the specifics, lacks the urgent earnestness on display from everyone else, and especially from Michael Craze, who plays Ben’s fear and panic with absolute conviction. Hartnell also bumps his head on the War Machine, which is quite amusing.

The episode only fails in the final defeat of WOTAN. It’s a one-take, as-live video sequence, absolutely the norm for 1960s TV programmes, and it is at least comprehensible, unlike the climaxes of some of the videotape Avengers episodes. But it’s a mess of Ben rushing in to extract Polly as Krimpton wrestles with a War Machine and Brett sort of hops about as small explosions go off. As the conclusion to Doctor Who‘s most action-packed story yet, it can’t help but look bathetic after Ferguson has done so much great work earlier in the serial.

And then, avoiding any tedious questions or tidying up, the Doctor just slips away as he did in The Savages. This is a new trend, more usually associated with the second Doctor, but clearly established here. The final scene unnecessarily reminds us of Dodo – who has decided to break up by the Sixties equivalent of text – before Ben and Polly barge into a dematerialising TARDIS. Two passers by stare at the camera, as if asking us, ‘Would you believe it?’ Thanks to Innes Lloyd and Michael Ferguson – yes, we probably would.

Next episode: The Smugglers