The opening scene of Before the Flood almost felt like it had been added after the fact, in order to warn the viewer ahead of any forthcoming sense of disappointment; “Don’t worry if you felt tonight’s story didn’t go anywhere, it wasn’t supposed to.” But pre-warning the viewer that they’re going to be underwhelmed sadly doesn’t compensate for the disappointment when it happens.

Not that the second half of Toby Whithouse’s two-parter was especially poor. Steven Moffat has set up Series 9 so that it is largely comprised of two-part stories in which the second episode works in some way to inform the first, rather than simply being a continuation of it; he does love to use the idea that there’s a week between instalments to play with people’s expectations, and to give them a sense that real time is passing in fictional time at an equivalent rate. The result of this, however, can be to undermine the symmetry of the storytelling if the latter half doesn’t pay off on the former in a consistent manner – and Whithouse’s story feels like this series’ first victim of the approach. Under the Lake was a very straightforward episode, setting up a mystery ostensibly in order to wrap it up, but Before the Flood sets out to do something entirely different, meaning the resolutions felt largely inconsequential and were somewhat lacklustre when they arrived; using the second energy pod to blow up the dam and flood the valley because the second energy pod was missing, the dam was bust and the valley was flooded isn’t an especially clever way to tie the pieces of your puzzle together; it doesn’t involve any pre-emption on the audience’s behalf, being neither something that they can be expected to guess at the reasons for in advance, nor something that comes as a surprise when it happens. That it was the Doctor in the suspension chamber was only a surprise because it was so obvious, and even though other elements of his plan might have been foreshadowed, such as the use of a hologram, they didn’t feel validated because the story didn’t give any compelling reason for their existence, nor any particular reason for the viewer to expect to have been deceived into missing what needed to feel like an inevitable explanation. Besides, this was the second story in a row to have cliffhangered on the death of a series regular, only for it to be revealed to have been a deliberate illusory ploy.

Toby Whithouse has always been great at writing character; less so with the science fiction. The reason Being Human was such a phenomenal series was because it concentrated on the effect that being a werewolf, a vampire or a ghost had on its protagonists, rather than dwelling too much on how these things could have happened. His previous Doctor Who stories have fallen into two categories; School Reunion and The God Complex are classics, largely because they relegate the sci-fi conceits to the background and focus instead on the people caught in the situation, while The Vampires of Venice and A Town Called Mercy work only slightly less well because the plots are just that bit more integral to the emotional beats, and therefore require a greater leap of faith in order to accept.

Before the Flood is something else. The conceit for this episode is a temporal paradox without either an entry nor an exit point – the more acceptable use of temporal paradoxes in science fiction involving the question of what caused the loop to happen and what it will take to unravel it, while this had neither – and it’s an idea that really needed to have been written by someone who was stronger at the conceptual stuff. As it was, we were presented with an episode that told us at the beginning – in an audacious and rather wonderful cold open – that no explanations were going to be offered, other than the pragmatic and rather prosaic explanations for how the pieces arrived in their positions in the future portion of the story, and then proceeded not to bother playing with that conceit. You can’t help but feel that Steven Moffat would have told you he wasn’t going to play a trick and then not only played it, but in such a manner as to deceive you that he had done so. The story itself is rather stodgy, then, when it might have been mischievous.

You can’t fault Whithouse’s writing of people, though, nor of his making the best use of the story he can tell. The underwater base, already used to magnificent effect in last week’s episode, became even scarier this week as the number of participants in this end of the story diminished – and the moment when Clara worked out why one particular character was immune to the ghosts, and therefore had to be the one to go in search of the MacGuffin, was brilliantly executed, all three of the players in the scene ringing true and emotionally honest. Emotional honesty is a gift of Whithouse’s, and these two episodes ended up much more than the sum of their parts because of it. Even if the story didn’t add up, the actors carried it because their parts had been written so well.

Less effective was the Doctor’s half of the story, with elements continually being introduced – that it was an experimental Russian village, the half an hour’s skip further back into the past, the Fisher King himself maybe – only for them not to be used. What did work, once again, was the emotional aspect – and the final payoff about the base commander and her signer, although a little pat in its execution, resonated for all the surviving characters in a way that the story entirely vindicated. There wasn’t a single character who didn’t feel genuine, nor one that felt superfluous, even Paul Kaye’s sadly all-too-brief turn as the Tivolian being both hilarious and yet perfectly consistent with – despite being ostensibly at odds with – the rest of the production.

Ultimately, Under the Lake and Before the Flood, when taken as a whole, must be considered the first failure of this series of linked episodes, for although each instalment introduced a number of ideas that would have been interesting enough to explore in themselves, taken together they neither of them paid off in a fulfilling way. Indeed, perhaps the problem was that this was a two-part story, because the base under siege element of the first half and the bootstrap paradox element of the second would have been strong enough to carry a shorter story without the need to try and tie the two together – and maybe this pair of episodes would actually have been better served as two single parters.