‘Crimson’ is very much horror in the Hammer aesthetic, except this time the roles are reversed and it’s the monsters who are vanquishing the humans; the damsels who are causing the distress, as the investigative squad of Madame Vastra, Jenny, and Strax journey to a land just as foreboding as Transylvania – the North (thunderclap, horse whinnies).

In place of a dark gothic castle there’s the dark Satanic mills of Victorian industry; instead of murdered peasants there’s murdered working class folk, found carmine-coloured and resembling victims of a tomato soup factory disaster. There’s even a creepy coroner with one foot in Gatiss’s old stomping ground of Royston Vasey who, with every guttural chuckle, adds his own tongue-in-cheek miasma to the atmosphere of horror. If Peter Cushing turned up as a cloth-capped whippet breeder you wouldn’t be surprised.

Who’s behind such manufactory-based malfeasance? None other than Dame Diana Rigg as Mrs Peel Gillyflower, who’s practising Eugenics with a capital ‘Eee Bah Gum!’ in her Saltaire-esque model town, the unsavoury Sweetville. Sliding effortlessly back into the accent of her birthplace, and clearly enjoying the cackling moral degeneracy of the mad old bat she plays, Rigg is one of the most entertaining villains the show has ever had, albeit far from the most threatening. Transcending the boundaries of fiction, she’s joined by her own daughter, Rachael Stirling, the emotional core at the centre of this whirlwind of stylised steam n’ scream shenanigans.

It’s always a pleasure to see Neve McIntosh return to her cold-blooded alter-ego Madame Vastra, but this time it’s the Silurian’s plucky wife Jenny (a marvellous Catrin Stewart) who steals the limelight, and with it no doubt many a heart. And as we adults deal with our feelings for two such wholly attractive fictional characters, Uncle Strax is on hand, like a great potatoey puppet, to keep the kids amused with his talk of Limbo Gas and grenades and triple-nonsense weapons. Much as Vastra has become the definitive Silurian for a generation, so Strax has repositioned the whole Sontaran military as a Scouts’ gang show rather than the Gallifrey-invading foe of old.

That fits in well here though, for ‘The Crimson Horror’ is Penny Dreadful pantomime; a tongue-in-cheek Victorian tale that drinks deep of the aesthetics and apocrypha of the age (right down to a high-definition-defying segment of film at one point) and isn’t afraid to chortle at itself and everyone around it.

Not only that, should anyone ever be wise enough to put a ‘Madame Vastra Mysteries’ spin-off into production, this is the template to use for it. Many a Whovian lives in hope that that’ll happen. Or ‘Whope’. Ooh, no. Sorry…

Airs at 6.30pm on Saturday 4 May on BBC One.

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Watch the Series 7 Part 2 trailer…

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