Proving that necessity is the mother of invention, this astonishing episode was thrown together on a shoestring budget when The Dominators was cut down from six episodes to five. The cast consists of four speaking parts, the only set is the TARDIS, and the monsters are a repaint of robots from Out of the Unknown. There’s the potential for this to go wrong: the last time the show tried to do a small, self-contained episode, The Wheel in Space 1, which in many ways this superficially resembles (the limited cast, the TARDIS fluid links blowing and its temporary destruction, the tempting images on the scanner, the threatening robot and the incapacitated Doctor), David Whitaker made a bit of a hash of it. This looks even more impressive in comparison.

Even the TARDIS looking its most threadbare (a mix of tatty photo blow-up walls, and plain flats) can’t distract from the power of the images writer Derrick Sherwin and director David Maloney throw at us. There’s a superb moment of horror where Zoe is tempted by the image of her home city on the scanner, leaves the TARDIS and vanishes – at which moment the picture of the city blinks out of existence as well. Later, she and Jamie are confronted by ghostly copies of themselves, dressed all in white to match the dead white space all around them.

But even if they’d stayed inside the TARDIS they wouldn’t have been safe. Right from the outset the Ship seems to be a less safe haven than it’s ever been before: threatened by being consumed by lava from the erupting Dulcian volcano; developing a fault, and then slipping out of time and space altogether. At which point, an unseen power begins to take control of its functions, and exerts a malevolent influence over the Doctor, who’s barely able to hold off the mental assault. Troughton shows his versatility with a performance of far greater gravitas than the last few weeks: gone is the clowning about. Instead, his serious warnings to Zoe suggest he’s genuinely frightened about what they’ve got into.

We could surmise that Sherwin is borrowing from Haisman and Lincoln, with a disembodied consciousness attacking the Doctor’s mind while its robot servants threaten his friends. Perhaps contemporary viewers wondered if this was the promised third meeting with the Great Intelligence on its own ground. The episode doesn’t give any answers. And it ends, shockingly, with the TARDIS exploding, and its contents spinning off into a void.

Next episode: The Mind Robber – Episode 2