Why does Susan Twist keep appearing in Doctor Who? All the theories

Doctor Who writers are known for playing the long game, setting the stage for big reveals further down the line and keeping viewers guessing in the meantime.

One mystery that is permeating through Ncuti Gatwa and Millie Gibson's first season together is that of actor Susan Twist's continued appearance on the show. The actor has portrayed a number of characters, coming across the Doctor and his companion at Abbey Road studios in the 1960s, on an alien planet, and even in modern-day Wales.

So why is the actor being brought back again and again on the BBC show? Let's examine the theories.

Why does Susan Twist keep appearing in Doctor Who?

Twist has played several background characters in the Doctor Who universe, first appearing in the 60th anniversary special Wild Blue Yonder where she played Mrs Merridew opposite Nathaniel Curtis' Isaac Newton. She then returned for a new role in The Church on Ruby Road and played different characters in Space Babies, The Devil's Chord, Boom, 73 Yards, Dot and Bubble, and Rogue.

The actor spoke on the matter with Doctor Who Magazine, joking: "It’s so nice to be these different people and have all those different looks. It’s very clever. ‘Ooh, it’s that woman again.'"

Twist will return in a more central role in The The Legend of Ruby Sunday, the penultimate episode of the new series as the Doctor and Ruby have finally addressed the character's regular appearance on the show. The character is set to take on the role of Susan Triad in the new episode.

Showrunner Russell T Davies had previously tried to explain Twist's reappearance in Doctor Who on the show's official podcast, dismissing any notion that she is playing someone important.

He said: "We ran out of actors actually, genuinely, there was a shortage at Equity. So we keep using Susan Twist for a lot of things, just because we like her. And that’s the simple reason why.

"There’s nothing behind it. No mystery, no nothing. She’s just there. I said, ‘Come on, Sue, come on, have a laugh.’ Yes, she keeps cropping up, faithful viewer. Yes, we’ve noticed. Yes, you’ve noticed."

That may be so, but Doctor Who fans didn't really believe Davies and his suggestion that Twist's ongoing appearance in the show is a result of an actor shortage. This has led to the rise of some interesting theories, and the hope is that all will be revealed in The Legend of Ruby Sunday.

Susan Twist will play The One Who Waits

The biggest theory around Twist is that she is really playing The One Who Waits, a character that has been teased on multiple occasions. The Toymaker first brought up the character in The Giggle, and Jinkx Monsoon's Maestro said in The Devil's Chord that the mysterious villain was "almost here".

Read more: Who is The One Who Waits in Doctor Who? All the best theories

What if The One Who Waits has been hiding in plain sight all along? And is simply waiting for the Doctor to notice them. Or perhaps they're lying in wait until they're ready to take on the Time Lord.

It'd be an interesting way to reveal the character and one that will no doubt come as a twist to many, because some viewers may have dismissed or not even noticed short cameos from a background character.

Susan Twist is Ruby's birth mother

Another potential explanation is that Susan Twist's character is really Ruby's mother, and the reason that she keeps popping up is that she is watching over her daughter and her adventures with the Doctor.

Read more: Every Ruby Sunday Doctor Who mystery and the theories that could explain them

There is a chance this is the reason, especially because the biggest mystery of the show is about Ruby Sunday and her parentage. So if the character revealed herself as being Ruby's mother then that is one way to put that mystery to bed.

Susan Twist is Susan Foreman

A deeper cut than the other theories, one explanation for Twist's appearance in the series is that she's really playing Susan Foreman — the Doctor's granddaughter and first companion. The Doctor mentioned Susan in The Devil's Chord, and that he wasn't sure if she had survived the killing of the Time Lords, but what if she did and the character continued to time travel without him?

This fan theory is probably the most out there of the explanations, namely because it relies on being a play on the actor's real name more than anything else. It also suggests that The Doctor doesn't recognise his own granddaughter.

That doesn't mean it's not a possibility though, as the Doctor said in The Devil's Chord, there is always a twist. All will be revealed in The Legend of Ruby Sunday, we hope.

New episodes of Doctor Who premiere at midnight every Saturday on BBC iPlayer, and then air on BBC One at 6.50pm.