Doctor Who air time confirmed for Dot and Bubble on BBC One

Little is known about Dot and Bubble at this time, but guest stars Callie Cooke (Henpocalypse!, Peacock) and Tom Rhys Harries (White Lines, The Split) are confirmed to appear.

The latest ratings for Doctor Who season 14 showed that 2.04 million tuned into last week's episode – simply titled Boom – as it went out, with that number expected to grow when iPlayer streams are factored in over the first seven days.

For comparison, the overnight ratings of the double-bill premiere – Space Babies (2.6m) and The Devil's Chord (2.4m) – rose significantly when consolidated with viewing across the first weekend, jumping to 4.01m and 3.91m respectively.

The season has been particularly effective at planting intriguing mysteries, from the snow that appears whenever Ruby (Millie Gibson) is in peril to the repeat appearances of Susan Twist that have fuelled speculation on who she could be playing.

Another surprise came in Boom, when Andor star Varada Sethu made her Doctor Who debut as Mundy Flynn, having been confirmed as a new companion for season 15 (in addition to Gibson's Ruby, rather than replacing her).

In response to the shocked reaction, episode writer Steven Moffat said: "There is a plan. That's all I’m telling."

On the other hand, Sethu's comments on Doctor Who Unleashed gave a more spontaneous impression of her (re-)casting.

"I was just in it for the ride, I had a great time for those three weeks," she said. "I was devastated to be leaving because I had so much fun but I didn't know anything about it. That call came much later, months and months later."

The appearance has already been compared to Jenna Coleman showing up as variants of Clara Oswald prior to her companion's introduction, but that wasn't the first time in revived Doctor Who when such a situation arose.

Freema Agyeman appeared in the season 2 finale as Adeola 'Adi' Oshodi, before stepping into the role of Martha Jones in season 3. Adi was later retconned to be Martha's cousin, with whom she bears a striking resemblance.

Perhaps the mystery of Sethu's Mundy could be solved just as easily? We'll have to keep watching to find out!

Doctor Who continues Saturdays on BBC One. Stream on BBC iPlayer. Check out more of our Sci-Fi coverage or visit our TV Guide and Streaming Guide to find out what's on.

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