That leaf has its starring role, in the first of many surprising decisions, in the pre-credits sequence, which plays out a tragic love story in the warm-your-cockles manner of a John Lewis Christmas advert. Monitoring events behind a pair of Cosmo Smallpiece specs and a copy of the 1981 Beano Summer Special, Matt Smith’s Doctor shows, yet again, a concern with impossible things and the childhoods of his female companions.

It’s a strange preoccupation, given that so many of them grow up to regard a once-round in his snogging booth as a wish-fulfilment fantasy – the equivalent of poring through the family albums of a lunatic date to discover how they became so freaky. But the awkwardness is mitigated by the knowledge that, in the child-centred world of Steven Moffat’s Who, there is no bigger child than the Doctor himself.

Setting foot in an intergalactic market, the Doctor is as geekily enthused as ever, wrapping his mouth round the names of kooky aliens – a Hooloovoo! – like the words themselves are new-fashioned on his tongue.

When he performs a haka of greeting with a saucer-eyed insect man, it’s with the incoordination of a man to whom humping, of any kind, is a foreign language. It’s funny and gauche, and certainly a better judged comic moment than the Doctor barking at Dor’een. But too much excitement can distract this most skittish of Time Lords, and he has soon wandered off, leaving Clara to play a game of hide and seek with Queen of Years, Merry (played by Emilia Jones, daughter of Aled).

Merry is on the run from steampunk-suited aliens, The Vigil, who are coming for her, ready or not, and it is her interplay with Clara which gives an often improbable episode its heart. In an adventure which is concerned with the power of stories, and particularly religious stories, to shape culture and identity, the conceit of a child raised as a vessel for the memories and heritage of her people is a good one. But Merry isn’t just a vehicle for story, she is also, like all frightened children, in need of one.

Thus both Clara and the Doctor take it in turns to play the role of bedtime storyteller, Clara drawing on memories of her late mother and the Doctor, typically, painting an account of the origins of the universe that is one part Richard Dawkins to two parts Lewis Carroll. Before the Doctor can say, ‘Believe in me’, Merry has channelled her inner Bonnie Tyler and gone out all lungs blazing in an intergalactic song festival – the purpose of which is to lull the old god, but which has precisely the opposite effect. A kind of Neuro-Frisson Song Contest, if you will.

Thematically, it all ties together. It is, fundamentally, a cleverer script than some of the gags make it out to be. And yet that it will be remembered as such is a story we cannot believe in – a possibility denied by the sentimentality of the climax, when the parasite god is himself defeated by story and the crowd of lump-faced monsters sways along to the song of Akhaten like a coachload of golden grannies.

The parasite, it turns out, is not a god – he’s just a very naughty boy – and as the Doctor begins to goad him with the story of his own arcane history, it is hard not to hear the series worshipping its own cult, retelling the one story that diminishes the more often it is told.

Untold stories, endings that should have been but never were: sometimes they come down to the smallest of production decisions – a joke here, a mask there. ‘Take it all, baby!’ the Doctor screams, and on a hundred keyboards, somewhere in cyberspace, the words ‘jump’ and ‘shark’ are typed. It is the kind of dialogue that can undo the grandest work from design and effects teams.

Clara produces from her book ‘the most important leaf in human history’ and Jenna-Louise delivers the line with such conviction, you want to cuddle her. It is the one story the episode is certain will intrigue: the one about the impossible girl. But it is too late. The story of Akhaten is part of us now, and, as adults, we are denied the privilege of childhood and playground to re-enact it, retell it, and make it better.

Aired at 6.15pm on Saturday 6 April on BBC One.

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