The next episode of Doctor Who features a surprising connection to a classic Doctor Who character.

Guest starring Poirot actor David Suchet as The Landlord, ‘Knock Knock’ airs at 7.20pm this Saturday.

The official synopsis reads: “Bill is moving in with some friends and they’ve found the perfect house – so what if it’s strangely cheap to rent, and the landlord is a little creepy? The wind blows, the floorboards creak, and the Doctor thinks something is very wrong. What lurks in the strange tower at the heart of the building – and why can’t they find any way to enter it?”

The episode sees one of Bill’s new flatmates, Harry, mention his grandad going backpacking with his boyfriend along the Great Wall of China.

Writer Mike Bartlett has revealed to Doctor Who Magazine that Harry’s other grandad is none other than UNIT surgeon, Harry Sullivan.

The former companion appeared alongside the Fourth Doctor and Sarah Jane Smith, and was played by actor Ian Marter.

Bartlett commented: “It was a reference that I enjoyed hugely, because I love Harry Sullivan and housemate Harry has, I think, a similar sort of attitude, of energetic sort of pluck.”

Unfortunately the line was cut from the final episode.

Bartlett explained: “It was decided that, in 2017, people might not remember one companion from 40-odd years ago, so it got cut.”

The Doctor Foster writer added: “People have amazing relationships with their grandparents, so it’s quite nice tapping into that. It’s not something we’ve seen too much in the modern series; the companions have always been either friends or sort of love interests. But with this they’re more like family, certainly in my episode.”

Pre-order the new season on DVD on Amazon here.

Pre-order the new season on Blu-ray on Amazon here.

Issue 512 of Doctor Who Magazine is out today.

Are you looking forward to ‘Knock Knock’? Let us know below…