Marking, Mike Bartlett's first writing credit for the series, "Knock Knock" sees Bill and some of her friends searching for the perfect student accommodation. Luck seems to be on their side when they're approached by an elderly landlord (David Suchet) who offers them an old house, big enough to accommodate all of them. Of course with this being "Doctor Who", nothing is quite what it seems and the young students aren't so much new tenants but unsuspecting victims of something dwelling within the walls of the house. Fortunate then of course that the Doctor just happens to be present as he was helping Bill move in, and is soon hot on the case.
With the past three episodes being of fairly strong quality; this latest entry marks a slight downturn although that's not to say it's dreadful. It's anything but, however you can't help but feel there's a whiff of being there see that about it. The whole Haunted House motif has been done before as far back as the 1989 classic story "Ghost Light" and with the more relatively recent "Hide" back in 2013. Of course because something has been utilized before doesn't mean something fresh can't be done with it, can't it? Well, yes in theory but with "Knock Knock" it takes an old trope and ultimately does really nothing particularly new with it. The concept of something alien living with walls and pouncing on unsuspecting victims was after all the basic premise of series eight's "Flatline". So far so unremarkable, and that's pretty much the best way to describe this outing. Peter Capaldi is as ever on top form as is Pearl Mackie who has been anything if not consistent since she made her debut on the show, with former "Poirot" star David Suchet putting in a blinder of a supporting performance as the enigmatic Landlord. In general the acting is the kind of standard you'd expect from a BBC production, it's really just all rather run of the mill.
It does score points I suppose in that it allows Bill, to prove her worth as a companion given her deductive reasoning, which leads to a slight twist in the tale which although mildly diverting doesn't come off as enough of a revelation to rise it's story above the so so. Compotently directed with decent enough special effects but a humdrum alien threat it just feels like nothing more than passable filler material, and really that's all that can really the best that can be said for it. The most intriguing thing that could be said for it was the stories coda which references back to the running plot involving the vault, and the unknown prisoner that Nardol (Matt Lucas) has been keeping tabs on while the Doctor is absent. With fan speculation running rife I and no doubt other fans will be waiting keenly to discover if out suspicions of who lies within are what we might suspect.