The resolution to the cliffhanger is a bit of a let down given it’s a repeat of the way the charging unicorn was overcome, but that’s about the only weak thing about this episode.

Rather than continually prolonging the story with yet another encounter with a fictional character, writer Peter Ling begins to accelerate towards the climax. The only new obstacle introduced is the Karkus, a superb original creation, oddly amalgamating Flash Gordon style villainy with comic strip sensibilities and a guttural German accent declaring, ‘You will be minced meat!’ He looks like something out of the Adam West Batman series, and in response Zoe slips into Emma Peel mode for a bout of light wrestling that ends up as the best companion moment since Vicki led a revolution on Xeros. 

While Trougton and Padbury have silly fun together (the bit where Zoe pops out from under the Doctor’s cape is wonderful), Jamie is faced with the return of the White Robots, last seen in the void between dimensions – which starts to draw the different elements of the story together. Once the Doctor and Zoe arrive in the Master’s citadel, the Doctor quickly pieces together what’s going on – and successfully argues for an audience with the Master, who turns out to be another Padmasambhava type figure – a fairly genial and harmless old man in thrall to some disembodied consciousness that years for human creativity. The most horrible thing about him is the little ball of spittle in the corner of his mouth, which keeps getting more obvious and disgusting every time the programme is re-released in a more restored version.

The final few minutes are astonishing. The Master reveals his creation is Captain Jack Harkaway, which sounds so close to Harkness that surely it can’t be coincidence. The Doctor is said to be ‘ageless’ existing ‘outside the barriers of space and time’, which is a much more mythic and powerful suggestion than most of the origin stories we’re going to get over the next 50 years. And the final cliffhanger – Zoe and Jamie crushed inside the pages of an outsize book as a representation of them being overwhelmed by the Land of Fiction – is one of the best ever.

Next episode: The Mind Robber – Episode 5