Knock Knock. Who’s there? Yes. Yes, he is.

The past looms large in “Knock Knock” as “Doctor Who” plunders horror tropes for an uneven but effective and emotional episode.

When Bill and her friends are offered the rental property of their dreams, they sign up as quickly as they can. The Doctor’s happy to help her move in but once he sets foot in the house, he realises that something is very, very wrong.

Very much an episode of two halves, the first half barrels along with a breezy, gleeful energy as it ramps up the foreboding and creepiness of the house and its secrets, again relying on the greatest of Who’s tricks: taking something mundane and everyday – creaking floorboards – and giving them a terrifying provenance. Once the sources of the house’s eerie noises are revealed, it’s only a pivot to a potently emotional finale which saves the story from being a profound anti-climax.

Once again, performances are key in elevating the material. Of course, Capaldi and Mackie are firing on all cylinders now but it’s in David Suchet that “Knock Knock” finds load bearing timber. Suchet provides the creepy landlord with an abundance of complexity and pathos, overcoming the characters somewhat sketchy motivations and actions. Even Bill’s new would-be flatmates, picked off one by one in true haunted house style, are immediately more likeable and real than the tick box characterisations which undermined the lacklustre spin-off “Class” and the episode definitely loses much of its momentum and atmosphere once the last of them has been disposed of.Ironically for a season which has so far delighted in allowing stories time to breathe and grow, the pacing is a real issue here and it might have worked much better as a two-parter (four episodes in old money), with there being more time between disappearances to build a sense of dread and get to know the characters in more depth so their demises would have more impact. The fast pacing of the disappearances and other spooky shenanigans also makes it seem really out of character for Bill to be so resistant to the Doctor’s suspicions that something strange is going on. As it is, it rushes through the good stuff in about thirty minutes and spends the remaining time on a slow but emotionally charged exposition of the ‘monster’. The ultimate resolution is at least satisfying, even if the egregious deus ex machina ‘everybody lives’ outcome feels like cheating.

Ironically for a season which has so far delighted in allowing stories time to breathe and grow, the pacing is a real issue here and it might have worked much better as a two-parter (four episodes in old money), with there being more time between disappearances to build a sense of dread and get to know the characters in more depth so their demises would have more impact. The fast pacing of the disappearances and other spooky shenanigans also makes it seem really out of character for Bill to be so resistant to the Doctor’s suspicions that something strange is going on. As it is, it rushes through the good stuff in about thirty minutes and spends the remaining time on a slow but emotionally charged exposition of the ‘monster’. The ultimate resolution is at least satisfying, even if the egregious deus ex machina ‘everybody lives’ outcome feels like cheating.

Among the fun touches such as using the TARDIS as a removal van, there’s a tonne of foreshadowing delivered here, from mentions of Time Lords in big robes and collars to the Doctor’s casual and then suddenly dismissive mentioning of ‘regeneration’ to Bill, not to mention how often the Doctor is referred to as a ‘Grandfather’.

Overall, “Knock Knock” is another strong episode in a strong season so far (“Smile” excepted) and even if I would have preferred far longer to be spent on the haunted aspect of the haunted house, I can’t begrudge the episode shaping itself around Suchet’s tremendous performance too much. And as for the teases of the vault – enough already. Show us what’s in there!