It’s that time of the week again which means that a brand new Doctor Who episode is just around the corner. It’s back to Earth – at least, the present day one – in Episode 4 as Bill and her friends move into their new house, which you’ll no doubt be pleased to hear is perfectly normal in every way…

Oh, apart from the creaking floorboards, old fashioned plugs, lack of central heating and the fact that it lies in the shadow of a mysterious, foreboding tower… to name but a few. Basically, who on Earth would live in a house like this?!

Well, Bill and her mates, apparently, but you can hardly blame them for overlooking its sinister surroundings, what with the state of today’s housing market. Looking on the bright side, though, the rooms are massive, the rent is cheap and, much to Bill’s relief, there isn’t a living puddle, weird robot or big fish in sight. In fact, if it weren’t for the cockroaches and everything else we just mentioned, it would pretty much be the perfect pad. And just what lies within that aforementioned tower, which they’ve been forbidden to enter under any circumstances for reasons that will become evidently clear by the end of the episode? Well, you’ll just have to wait until the end of the episode, won’t you?

Let’s not get ahead of ourselves, though, as Knock Knock is an episode that’s less about the story and more about how the story is told. As with all classic adventures, it’s built on a simple premise – in this case, a group of students moving into their new digs (what could possibly go wrong…?) – that utilities the classic horror conventions to deliver the sort of spine-chilling thriller that only the weird and wonderful world of Doctor Who can offer.

There is always a danger that episode’s with such a straightforward concept will fall flat, but that definitely isn’t the case in this instance. While the majority of the action takes place within the four walls of Bill’s eerie abode, the story unfolds in such a tantalisingly terrifying manner that it never feels stale or predictable. That’s largely down to first time Who writer Mike Bartlett’s snappy dialogue, which is charmingly complemented and effortlessly excelled by Bill Anderson’s subtle but sophisticated direction. It’s a completely different tone than his previous work in last week’s Thin Ice, but he amicably adapts to the genre at hand to make Knock Knock one of the most atmospheric and genuinely heart-racing episodes to date. Seriously, this one is creepy.

Once again, it all comes down to the simplicity of the plot, combined with Doctor Who’s already firmly established ability to turn the seemingly mundane and everyday into a living, breathing nightmare… made of wood. What we also love about Knock Knock, though, is that it still provides room for the Doctor and Bill’s relationship to develop amidst the escalating terror. The Doctor – in keeping with his fatherly/grandfatherly/tutor/friend duties – helps Bill move in, which we thought was a lovely touch, and it turns out that she still has plenty of questions about who he is. Oh the questions, the questions, the questions…

Bill is more than capable of standing on her own two feet, of course, which is affirmed – and then reaffirmed – by her growing frustration when the Doctor decides to sticks around uninvited. He’s never been able to take a hint, but it’s refreshing to see a companion who is so eager to establish the bit of her life that the Doctor isn’t part of. She probably would’ve done a pretty good job at initialising her independence (much better than her predecessor Clara ever did, anyway…) if her friends hadn’t started disappearing into the walls, so something tells us that she was secretly over the moon that the Doctor was on hand when the lowering Landlord came to call.

Speaking of whom, David Suchet is one of the best Doctor Who guest stars ever, by the way. Not that we were expecting anything less from the honourable Hercule Poirot himself, of course, but his formidable on screen presence takes his character to a whole new level. He might just make you cry, too, in his scenes with the magnificent Mariah Gale as Eliza, but to tell you anything more about their unique connection would ruin the episode’s bittersweet – and heartbreakingly human – climax.

The rest of the ensemble are great, too, particularly Bill’s diverse group of friends – Shireen, Harry, Paul, Felicity and Pavel (don’t worry about remembering their names – the Doctor certainly doesn’t!) – who each react to the drama at hand in a different way. Of course, the episode wouldn’t be completely without a cameo from Nardole, and you’ll no doubt be pleased to hear that he’ll play a much bigger part from this point onwards, if the preview trailer for next week’s Oxygen is anything to go by.

That bloody Vault returns in Knock Knock too, as the Doctor takes the mystery one step further. Hopefully it’s a step in the right direction, despite the fact we still know nothing about what – or who – lies inside. Apart from the fact that they are exceptionally gifted on the piano…

We can’t wait to find out what’s really going on, but at the moment we’re still enjoying the excitement of the enigma. Play on, you unidentified Vault-dwelling menace. Play on.