What is the big mystery surrounding Doctor Who's Wild Blue Yonder?

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We've also seen a fair amount come out around the trilogy closer, The Giggle – it'll feature the return of the villainous Toymaker, last seen on television way back in 1966 and now played by Emmy-winning actor Neil Patrick Harris.

Though not officially announced, trailer footage and interview teases have also pointed to appearances from UNIT, their leader Kate Stewart (Jemma Redgrave) and their scientist advisor Shirley Anne Bingham (Ruth Madeley).

But we know next to nothing about episode two, Wild Blue Yonder, which airs this Saturday (2nd December) – showrunner Russell T Davies has called it "weird" and "scary", but the brief plot synopsis that's been released is impressively vague: "The TARDIS takes the Doctor and Donna to the furthest edge of adventure. To escape, they must face the most desperate fight of their lives, with the fate of the universe at stake."

Besides Tennant and Tate, only one additional cast member has been confirmed – Susan Twist, who'll play a character named "Mrs Merridew".

The key art for the episode (see above) suggests the inclusion of a robot, though it's unclear whether it'll be friend and foe.

The trailer released shortly after broadcast of The Star Beast, meanwhile, also seems to delight in keeping things ambiguous, revealing only that the Doctor and Donna have landed in a mysterious space-age location, where they encounter "life signs" and are menaced by an unseen threat...

Doctor Who fans, of course, abhor a vacuum, so the lack of information about Wild Blue Yonder has sparked rampant speculation. Why the secrecy? What's being hidden?

Fan theories have included suggestions that the episode is set entirely within a TARDIS – with the Doctor's ship landing inside another time/space machine – or that all this cloak and dagger is concealing a huge surprise return, with names put forth including Carole Ann Ford as the Doctor's granddaughter Susan and Matt Smith as the Eleventh Doctor.

There's also a school of thought that the episode will feature an early appearance from Ncuti Gatwa's Fifteenth Doctor, and even Millie Gibson's new companion Ruby Sunday.

We know for a fact that the events of Flux, the Doctor Who series arc aired in 2021, will be dealt with "very slightly" in the episode, which has led fans to guess at everything from some involvement from The Division to a story set during "the Dark Times" (an age of chaos and monsters that predated the Doctor's reality).

To be clear, all of this is just guesswork at this point. In fact, Tom Kingsley, director on Wild Blue Yonder, has sought to play down the rumours while acknowledging that the plot is "a TOTAL SECRET".

Describing the episode as "weird and exciting", he added: "Fan PSA: the story of Wild Blue Yonder isn’t a secret because there are any surprise returning actors or villains. It’s just because we thought you might find it fun to watch it without knowing what’s going to happen next. Promise!"

Couldn't that rationale apply to any other episode, though? Adventures that weren't/aren't being kept under wraps to the same degree?

Perhaps the biggest clue we have to latch on to comes via David Tennant. Speaking to Total TV Guide, he said: "The second script is unlike any Doctor Who I’ve ever seen.

"I don’t want to give too much away, but I was always aware of the casting that was going to be part of the episode. That was something that Russell was very specific about. I just didn’t imagine it would… manifest itself in the way that it does..."

Could this piece of casting be the cause of all the covertness? If we take director Kingsley at his word that Tennant and Tate will be the only series veterans appearing, then perhaps it's not a returnee we should be expecting but some other familiar face – a big name actor whose casting is yet to be announced? Or just a particular type of performer?

Clearly, whoever it is, or whoever they are, said casting has a significant impact on the plot...

"I love humans," the Eighth Doctor once commented. "Always seeing patterns in things that aren't there," - it does feel, however, as though something is definitely afoot here.

Just three days to wait until the exact nature of that secret is revealed...

Read more:

• First look at Doctor Who's new Eighth Doctor audio adventure cover
• How Doctor Who's epic Wrarth Warriors were brought to life
• Ncuti Gatwa and Millie Gibson in first look at Doctor Who Christmas special

Doctor Who continues on Saturday (2nd December) at 6:30pm on BBC One. Previous episodes are available on BBC iPlayer and on BritBox – you can sign up for a 7-day free trial here.

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