The Quatermass inspirations remain pretty clear in this episode. Apart from Noah’s Carroon-like transformation, and the squirming Wirrn in the solar stacks that recall the aliens in the vats in Quatermass II, we get the Doctor’s version of Quatermass and the Pit’s optic-encephalogram, a device that allows him to relive the history of the Wirrn Queen’s infiltration of the Ark. It’s a good sequence, but the really disturbing thing about it is the Doctor’s reaction: mentally linked to the Wirrn, he seems subsumed by its intelligence, to a point where he walks towards a menacing grub, as if seeking to join with it, and has to be physically restrained by Sarah Jane. All this cosmic horror is entirely Nigel Kneale (or at least Hammer’s full-bodied adaptations), much more so, in fact, than most of Season Seven.

Equally, Robert Holmes’s style comes through quite strongly. It’s a shame Lycett and Rogin don’t get longer onscreen together as they definitely have the potential to make a double act (I half suspect Steven Moffat was thinking of them when he wrote the Fat Gay One and the Thin Gay One in A Good Man Goes to War). Rogin in particular, masking his fear with sardonic humour and exaggerated world-weariness, is a very particular Holmes type, a kind of relatable throwback like Milo Clancy in contrast to his sterile crewmates.

Most of this is strong. Opening the episode with a message from the long-dead High Minister of Earth gives an opportunity to recap on the premise of the story and what’s a stake. Playing it over a sombre Vira, a facetious Harry and Noah battling for the remnants of his own humanity, playing out in miniature the great struggle outlined by the Minister, makes a powerful sequence in a serial that isn’t short of them. Only the direction lets it down at points – particularly in Harry and Rogin’s encounter with a horribly mutated Noah as their fission guns seem to be replaced by a wall-mounted gun in one shot.

Next episode: The Ark in Space – Part Four