Doctor Who fans, here's an amazing Easter Egg for you: one of the characters in this Saturday's episode is related to a classic series companion.
'Knock Knock' sees Bill (Pearl Mackie) and a gang of friends move into a grand manor house that's concealing a very dark secret.
One of her pals – Harry (Colin Ryan) – tells Bill that his grandfather went backpacking in Asia with his boyfriend, and was almost arrested for trying to steal a piece of the Great Wall of China!
[Above: Colin Ryan as Harry in 'Knock Knock']
But, speaking to Doctor Who Magazine, the episode's writer Mike Bartlett revealed that 'Knock Knock' originally featured a reference to Harry's other grandfather... who is none other than Harry Sullivan.
For the uninitiated, Harry (played by Ian Marter) travelled in the TARDIS with Tom Baker's fourth Doctor in the 1970s.
"It was a reference that I enjoyed hugely, because I love Harry Sullivan," Barlett said. "And housemate Harry has, I think, a similar sort of attitude, of energetic sort of pluck."
[Above: Ian Marter as Harry Sullivan]
Unfortunately, the reference to Harry Sullivan was cut from the final episode. Boo!
We do know a little of what happened to the character after he opted to give up travelling in time and space for a quieter life, though.
In a 2010 episode of Doctor Who spin-off The Sarah Jane Adventures, Sarah Jane (Elisabeth Sladen) mentions that Harry continued working as a doctor, saving "thousands of lives" with his vaccines.
You'll have to watch 'Knock Knock' this Saturday at 7.20pm to find out what happens to his grandson, Harry mk.2...
Doctor Who Magazine issue 512 is on sale now.
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