The Telegraph thought the acting plaudits belonged to the main guest star. "David Suchet playing the Landlord was brilliant casting. Just menacing enough as he mysteriously appeared without warning, smiling evilly in corners. Grey-haired and brown-suited, Suchet even looked ghostly. Never before has a tuning fork seemed so sinister."
Radio Times relished the creepy nature of the episode, but was disappointed by the ending. "For the first 35 minutes or so of Knock Knock I’m enthralled. Unfortunately, it showed signs of collapsing like a soufflé during the denouement...... I’d have been far happier with an unhappy ending. And yet I’m dismayed that, as the house crumbles, Bill loses all her knick-knacks, including the treasured photos of her mum that the Doctor sourced for her only weeks ago.
Den of Geek takes the opposite view, preferring the last third of the story. "Appreciating that some aren’t happy when Doctor Who goes emotional in its story wrap-ups, the fact that David Suchet’s mysterious landlord is actually the son of the woman we assumed to be his daughter is a very well earned, impactful piece of storytelling. I like that it works as a twist, but mostly, that it works because it adds real tenderness to the story."
The Mirror felt the episode to be the weekest of the season so far, feeling the character of The landlord was confusing. "Rather than allowing the bad guy of the week to be wonderfully creepy, he gets a 180 in the last few minutes to try and make us sympathise with him. But again, you can’t go killer madman to poor boy trying to save his mother in 3 minutes flat. Frankly, if you bag an actor of such calibre you can't waste him on a character that chops and changes."
Games Radar felt the writing was disapointing after last week's triumph. "The flat story is elevated slightly by David Suchet’s masterful acting, and Bill coming out as bluntly as possible does provide an air-punch moment. But even those stand-outs can’t redeem the overall limp writing. "
Flickering Myth disagreed. "When Doctor Who aims to frighten you, it really knows where those dark corners are in your mind, where you keep those little things you don’t like in your own cozy home, and brings them into light for everyone to see and to watch you leap two foot off your sofa. In this latest episode of Series 10, written by Mike Bartlett, you’re seriously going to be considering screwing down that creaking floorboard outside your bathroom by the end of it."
Carter Matt also enjoyed the story. "What was so fascinating about this story was that the more we saw of it, the more apparent that it became that this was not a story about good and evil; it was a story in some ways about a sacrifice, and what a father was able to do for his daughter. "
AV Club appreciated the style of the story. "Rarely does Doctor Who embrace another genre as completely as the first 30 minutes of “Knock Knock” does. For the first two thirds of its running time, tonight’s episode is a horror movie, with all the customary trappings. There’s the spooky haunted house with the still spookier landlord. There’s a bunch of thinly sketched young people who are all possibly idiots. There are loud noises and eerie music and windows and doors shutting all by themselves. If anything, it’s all a bit old-fashioned, in the best possible way."
Ars Technica praised the effects in the episode. "The storyline is a bit horror-by-numbers as one by one Bill's pals are bumped off by the alien woodlice. The on-screen effect is pretty grim, if thoroughly gripping, and reminds me of the carnivorous scarabs in The Mummy."
Digital Spy also felt the effects were well realised. "The episode is admirably daring when it comes to its scares, throwing up some genuinely disturbing visuals, particularly the sight of a pained Pavel (Bart Suavek) half-eaten by the house, and of a screaming Harry (Colin Ryan) being eaten alive by the Dryads."
Screen Rant looked at the themes behind the current series. "Weighing the life of one against the lives of many has been a recurring theme during Capaldi’s tenure as the Doctor, and with ‘Knock Knock’ the series places the Time Lord in a more intimate situation, almost like it’s a test to see what he’ll do when faced with a group of twentysomethings trapped in what appears to be a haunted house"
Nerdist also picked up the horror in the story. "One of the show’s main goals since the beginning was to scare people senseless. Monstrous aliens are part and parcel of the show’s success, and that often sends people “behind the sofa.” This week’s “Knock Knock” fits right in that genre…for good and ill."
IGN returned to the relationship between The Doctor and his new companion. "I enjoyed “Knock Knock” from the get-go, particularly -- as usual -- due to the Doctor’s interactions with Bill, as we see her here slightly embarrassed by the weird old fella around her new friends. This is her granddad helping her move, she insists, but no, he says, he’s her dad. Can’t we at least say I’m the dad?"
Link to Doctor Who News Review