Doctor Who airs a major development in Susan Twist mystery

Alright, so technically Ruby previously recognised a hiker played by Twist as someone she'd met before in last week's 73 Yards, but those events were later erased, meaning that her and the Doctor's later realisation in Dot and Bubble is highly significant.

Dot and Bubble sees the Time Lord and his companion attempting to virtually guide Lindy Pepper-Bean (Callie Cooke) out of the monster-infested residence of Finetime using her "dot and bubble" communications system – along with Lindy, they watch back a message from her mother Penny Pepper-Bean, played by Twist.

(Penny, in case you were interested, is aged 62 and has 3,108 subscribers in her friendship circle.)

"We miss you so much darling, but I'd pay for the whole moon to make you happy," Penny tells Lindy in the message. "Just look up at the sky and wave to us on the homeworld. Happy fine time, Lindy-Lou... and don't forget, you're only a bubble away."

It's clear watching the message back that the Doctor and Ruby both recognise 'Penny' – "I've seen her before," insists Ruby, with the Doctor making the link between Lindy's mother and "the face of the ambulance on Kastarion 3" as seen in previous episode Boom.

"No no no, I've seen her somewhere else," says Ruby, but with Lindy still in danger, the Doctor advises, "We'll save that for later."

Twist's role in Dot and Bubble is notable for being the first time she's played someone with a familial link to another character in an episode, suggesting a life and history for 'Penny' – previously, she's portrayed more peripheral characters, but does her latest appearance imply that rather than being one character who is popping up in various disguises, she's actually playing different people across the universe who all share the same face?

The moment also recalls a scene in 2005 episode Bad Wolf, in which the Ninth Doctor (Christopher Eccleston) and Rose (Billie Piper) first realised that two words – "bad wolf" – had been following them in their travels across time and space.

That enigma was explained in the two-part finale that followed, and with Twist looking set to play a larger role in Empire of Death – the second episode of this year's closing two-parter, airing 22nd June – it looks like we could be in for a similar reveal there.

Doctor Who showrunner Russell T Davies has previously teased that Empire of Death will be "literally, the biggest finale ever" and also advised that fans should stay up until midnight to watch it if they can, to avoid falling victim to spoilers.

"Try to stay unspoilt, because it's so hard to be unspoilt with things," he said. "Try, try, try, and you will have a whale of a time with that one."

Doctor Who continues on Saturday 8th June on BBC iPlayer and BBC One. Previous seasons are available to stream on BBC iPlayer.

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