Four episodes into a series of Doctor Who feels like a reasonable time to get a pretty standard approach. Despite being able to go anywhere and play in various television genres Doctor Who has a bag of tricks it goes to fairly often. You need a format, even a hazily defined one, in order to make the big finales and episodes that play against expectations have an impact. You can't play against expectations if there are no expectations for a routine episode, after all.
This isn't an inherently bad thing. Just because something is adhering to a familiar format doesn't mean it can't do anything new, and generally TV shows develop and stick to formats because they work. Which is to say that Knock Knock wasn't a bad episode because it was a standard outing for New Who. It wasn't, actually, a bad episode at all. It's just that it wasn't a particularly good one either.
This is frustrating. There were a number of little things in this that could have been brought together in an interesting way. The Doctor taking a vaguely McCoy era approach to letting the companion wonder off into trouble. A child desperate to keep a parent alive, even if it means a bit of human sacrifice. A house that eats people. Giant woodlice that can convert matter1 who have infested a woman and kept her alive for sixty years. These were all good concepts, they just suffered from being strung together in a rather understated, by-the-number fashion.
Nothing was underdeveloped. It all hung together well enough as a plot and cohesive design and direction. But things could have gone further (or at least the things unique to this episode could have done, it's not fair to say the Twelfth Doctor could have gone further in his treatment of Bill because we don't know what's coming with them in future episodes). For example, if there's a walking, talking human (as opposed to a still prop) made of wood in your script that's probably going to be the most visually impressive thing in your episode. I understand she was confined to a lone set for an explanatory scene because of the nature of her character and to reduce the hassles of filming her but the episode could have been more interesting had she been the one stalking the students through the house instead of CG woodlice. Increasing her part and changing the nature of her role would have meant increasing the hassle of getting the actress in and out of what was probably an infuriating costume and makeup, I get that. But this could have been gotten around, in part, by shooting her in shadow, perhaps creating a prop to use in places. Or, y'know, by not showing her at all and implying her presence with creaks and knocks. It was meant to be a creepy episode about a haunted house. A wooden ghost that can't be reasoned with fits into that genre far better than giant insects.
That's not a major complaint. It's more an observation of how existing elements of the episode could have been reworked in a more interesting way. Peter Capaldi and Pearl Mackie were both very good. I haven't gone back and made comparisons but it certainly felt like each gave their best performance of the series so far. Poirot was good as the Most Significant Guest Character, even if it is weird that writer Mike Bartlett went out of his way not to give the character a name despite that character's own mother also being in the episode. You'd think she'd use her own son's name, right? I suspect Bartlett just liked having a character with a title, the Landlord, instead of a name because this is a show that has a strong tradition of such characters2.
Knock Knock could have been more than the sum of its parts had it set out to be. But it didn't so it wasn't. This was standard Doctor Who, which is what it set out to be. I suspect that's what the next episode is going for too. In which case, fine. Three generic approaches in a row isn't the worst thing in the world if all three succeed (and while Knock Knock and Thin Ice are both take standard approaches they're different standard approaches, with next week's Oxygen looking like it will be another3). But at some point this series is going to need to aspire to more, because it would be a shame for a promising Doctor-companion combo to get stuck in mediocrity.
1 By implication they can also restore consciousness and memory, which is even more incredible when you think about it.
2 Time for a bit of baseless Moffat-hating speculation: maybe Moffles suggested not naming the character in the hopes of people picking over previews and suggesting he was a rogue Time Lord or a(nother) new incarnation of the Master or Chibnall's future Doctor or something. It's exactly the sort of thing he'd do. He's not been referring to the monks we'll be seeing in a few episodes time as "meddlers" for nothing, you know.
3 Specifically base under siege.