Like Roald Dahl’s story, ‘Journey’ is a fantasmagorical tour of a place that’s half imagination, half impracticality, and half danger (yep, three halves – like the TARDIS we’re thinking on a dimensionally transcendent level here). Wisely, writer Steve ‘Sherlock killer’ Thompson doesn’t get too side-tracked by this most magical machine but instead lets the audience’s fascination with the mysterious interior of the Gallifreyan gadabout box become the thrust of the story.

It means that all at once we are the salvage-seeking Van Baalen Bros. (led by Outcasts‘ Ashley Walters) wanting to cut our way ever deeper to see what treasures this blue box holds; we are Clara, running into nostalgic props and oft-mentioned rooms; we are The Doctor, knowing full well that this is a living miracle which can hold at bay the assembled hordes of Genghis Khan, but which is also a deadly labyrinth of such architectural absurdity that MC Escher would suffer a nosebleed imagining it.

Much of the TARDIS’s guts are corridors, but that seems fitting given the life The Doctor’s lived. And yet, unlike the utilitarian tubes in ‘The Doctor’s Wife’, there’s more than just thirty-odd feet of plywood and paint. And much more than a swimming pool.

We won’t spoil anything, but there are some truly spectacular eye-opening (ahem) sights on offer that’ll excite new and old fans, as Thompson adheres to the years of Doctorly asides about the ship, but also brings new mythology to the old Type 40 TT.

Despite the twin wonders of set design and CGI on show, the TARDIS is still left largely to our imagination – a conceptual chocolate factory of sci-fi sweetness – and Thompson should be applauded for his restraint and focus. Faced with the borderless sandpit of the interior, lesser writers might squander story space indulging in possibilities.

No doubt in the TARDIS there’s a room made of jelly babies and bubble wrap, where you can perform karaoke renditions of the songs of the Ood to the battered remnants of Kamelion, but you won’t see it here, and rightly so. The last thing we want is to have all the ship’s mysteries stripped apart.

Only in one spot does the plot mirror the architecture of the TARDIS: a pocket of character nonsense that you simply have to shake your head and shrug and smile, because this is Doctor Who and we’ve grown into such madness. But this is a smartly-executed story, and in effortlessly packing the infinite into 45 minutes, ‘Journey’ shows what fans have known for 50 years: good things come in little blue boxes.

Airs at 6.30pm on Saturday 27 April on BBC One.

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