*Warning: Major spoilers ahead for Doctor Who season 14 episode 3, Boom.*

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Well, that was a surprise! Varada Sethu has officially made her Doctor Who debut much earlier than anticipated in season 14 episode 3, Boom.

The new episode, written by former showrunner Steven Moffat, has had audiences enthralled, not least because of its explosive story, which sees the Doctor arrive on a planet called Kastarion 3 and accidentally stand on a landmine, unable to move for almost the full episode. Classic Moffat.

But we had one other surprise in store – Sethu being cast as new character Mundy Flynn. Sethu wasn't supposed to make her first appearance in Doctor Who for another year, in season 15 of the show. So what exactly has happened here?

Speaking on Doctor Who: Unleashed, Sethu revealed that, when filming Boom, she had no idea she would later be cast as a full-time companion.

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She said: "I was just in it for the ride, I had a great time for those three weeks. 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."

Here's everything we know so far!

Who is Varada Sethu's character Mundy Flynn in Doctor Who?

Varada Sethu is introduced early doors in Boom, as an Ordained Anglican Marine named Mundy Flynn.

She clearly has a close bond with fellow marine Canterbury James Olliphant, or Canto, played by Bhav Joshi. She's also a trusted friend of Splice Alison Vater, and is someone Splice leans on following the death of her father, John Francis Vater.

We don't know much else about Mundy yet, but her name does prompt a lot of questions. Pronounced similar to "Monday", it seems to be intriguingly linked to Ruby's surname, Sunday, with the Doctor even joking about it in the episode ("Great name, Mundy Flynn. You should marry Ruby, then you’d be Mundy Sunday").

Ncuti Gatwa on the set of Doctor Who episode Boom
Ncuti Gatwa on the set of Doctor Who episode Boom BBC Studios,James Pardon

The episode begins with Mundy trying to convince Canterbury, who's clearly in love with her, to swap shifts with her – and he reluctantly agrees. Mundy heads off with Splice, and we don't see the pair again until later in the episode, when Splice hears a hologram of her father talking to the Doctor.

She runs after him, with Mundy following her, and Mundy quickly attempts to take control of the situation.

Pointing her gun at the Doctor at the worst moment – when the landmine's countdown enters its final stages and makes him explosive – she causes his blood pressure to rise, risking triggering the landmine. She tells the Doctor that she believes the Kastarians, who the Anglican Marines are supposedly fighting, are in the air, or in the mud, with the Doctor ridiculing the suggestion.

Ncuti Gatwa in Doctor Who standing on a green landmine looking shocked
BBC Studios/Bad Wolf, James Pardon

She quickly realises that the Doctor is telling the truth, and that if he explodes with the landmine, he'll cause the whole planet to explode. After the Ambulance (played by Susan Twist) detects combat, Mundy tells Ruby to shoot her so as the Ambulance can detect the shot and it can triage, crucially preventing the Doctor from being blown up – but before she can, Canterbury arrives and misreads the situation, shooting Ruby.

The Doctor tells Mundy the only way to switch off the landmine is to surrender – because the only thing they're actually fighting is an algorithm maintaining an 'acceptable' casualty rate and their own hardware, not the Kastarians.

With Canterbury arriving to help, he admits his feelings to Mundy – but before she can reply, Canterbury is shot dead by the Ambulance as the algorithm starts to fight back.

In a heartbreaking moment, his hologram appears, telling Mundy that he loves her and it's okay that she doesn't love him back.

The final scenes of the episode show the hologram of John Vater fighting the algorithm for his daughter and prompting a surrender, allowing the Doctor to finally step off of the landmine.

Ruby and the Doctor say their goodbyes to Mundy and Splice, but the Doctor says something that suggests it won't be their last meeting after all: "I will be popping in every now and then. Fish fingers and custard is my favourite."

Who will Varada Sethu play in Doctor Who season 15?

So, with Mundy Flynn left alive after Boom, will Sethu return as the same character?

Judging by the end of the episode, it seems the Doctor may return to Kastarion 3 to see Mundy and Splice again – and perhaps that's when she joins Team TARDIS to make it a trio.

But nothing is certain when it comes to Doctor Who. When she was cast, the BBC did not confirm her character name – so could it change?

Varada Sethu
Varada Sethu. Dave J Hogan/Dave J Hogan/Getty Images)

Sethu has also been seen in various photos on set looking very different to how she looks in Boom, particularly during one episode where she wears a yellow dress.

Of course, it might just be that she's delved into the Doctor's spectacular wardrobe – but could it be a situation similar to Susan Twist, where we keep seeing the same character across different time periods? Maybe something timey-wimey is going on and Sethu will also be playing various different characters.

Only time will tell for sure!

When will Varada Sethu be back in Doctor Who?

Officially, Sethu has been confirmed as a companion alongside Gatwa and Gibson for season 15, which is set to air in 2025.

But could she be back sooner? At this point, nothing is certain - although from what Sethu has said on Unleashed, we're not expecting her back now until next year.

But one curious point to mention is that Moffat, who wrote Boom, has said that he's already read the season 15 finale script "for reasons that will become apparent" in a recent interview with TV Choice magazine.

There could be any number of reasons for this – but one of them could revolve around Sethu. After all, Moffat is now the writer who's introduced Sethu into Doctor Who.

Maybe Russell T Davies also wants his input on how her story should continue.

Sethu's casting was confirmed in April. She said at the time: "I feel like the luckiest person in the world. It is such an honour to be a part of the Whoniverse, and I'm so grateful to the whole Doctor Who family - because that is what they are - for welcoming me with open arms and making me feel so at home.

"I couldn't ask for a better team than Ncuti and Millie to be on this adventure with, this is SO much fun!"

Davies added: "I first worked with Varada on a BBC production of A Midsummer Night’s Dream, and it's a joy to welcome her on board the TARDIS.

"Right now in the studio, shooting for 2025, we've got Ncuti, Millie and Varada fighting side by side - we need all three because the stakes are higher than ever!"

Doctor Who returns with 73 Yards on 25th May on BBC iPlayer and BBC One.

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