Warning: Spoilers ahead for Doctor Who season 14, episode 3, "Boom."

Summary

• Millie Gibson's Ruby Sunday will depart at the end of Doctor Who season 14, with a new companion appearing in season 15.
• Varada Sethu, who debuted in season 14, may become the new companion in Doctor Who season 15.
• Sethu's potential role as Mundy Flynn could add an exciting dynamic to the show's upcoming season.

Millie Gibson's Ruby Sunday will cease to be the Doctor's companion at the end of Doctor Who season 14, and the incoming actor who will take her place in season 15 has already appeared in the Disney era of the show. Despite being a mainstay of the Doctor Who season 14 cast, Gibson's character is set to be replaced at some point - presumably near the end of the current run of episodes. Although she is set to depart the show, showrunner Russell T Davies has already confirmed that Gibson will reprise her role in Doctor Who season 15.

Gibson's eventual return will reunite her with Ncuti Gatwa's Fifteenth Doctor, and also possibly place her alongside the incoming companion actor. At present, the reason for Ruby Sunday's Doctor Who departure has yet to be revealed, but it is seemingly not an exit that will be permanent. Thankfully, Gibson's future absence from the show appears to be one that's been planned for narrative reasons rather than as a result of behind-the-scenes drama. Either way, Doctor Who's Disney era will start its first two seasons with a different Doctor/companion actor pairing.

Doctor Who's Mundy Flynn Is Played By Varada Sethu, Season 15's New Companion

Sethu debuts in Doctor Who season 14, episode 3, "Boom"

The Fifteenth Doctor's new companion in Doctor Who season 15 will be played by Varada Sethu. As yet, her character name is unconfirmed. However, the actor's debut on the show came in Doctor Who season 14, episode 3, "Boom." Written by former Doctor Who writer/showrunner Steven Moffat, "Boom" serves as a soft sequel to other Moffat stories that feature the militarized Church that he introduced during Matt Smith's reign as the Eleventh Doctor. Although no members of the Church in "Boom" are returning figures, the character of Mundy Flynn is portrayed by Varada Sethu.

Little is known about Sethu's Doctor Who season 15 character, or whether they'll be connected to the actor's persona in "Boom."

Doctor Who season 15 currently has no official release date, but production has already begun. As such, it's expected to arrive at some point in 2025. If so, that means that Varada Sethu will have debuted on Doctor Who an entire year before becoming a member of the main cast. Little is known about Sethu's Doctor Who season 15 character, or whether they'll be connected to the actor's persona in "Boom." Although, given the nature of a mind-bending sci-fi like Doctor Who, it's certainly possible that there is a link between both Varada Sethu characters.

Doctor Who Has A Long History Of Casting Guest Actors As Future Companions

Sethu wouldn't be the first companion actor to cameo as a different character

Varada Sethu isn't the first actor to appear in Doctor Who before being cast as the Doctor's companion. For example, Freema Agyeman plays Martha Jones throughout Doctor Who season 3 and also made some guest appearances as Martha in Doctor Who season 4. However, her earliest role on the show came in Doctor Who season 2, "Army of Ghosts," where she played Adeola Oshodi. Adeola was a Torchwood One employee who perished in the same story, and she was retroactively revealed to be Martha's cousin. By doing this, the show rectified the pothole of both characters sharing the same face.

Similarly, Karen Gillan's most famous Doctor Who character is Amy Pond, the first companion of Matt Smith's Eleventh Doctor. However, Gillan appeared on the show before she stepped into the shoes of Amy Pond. In Doctor Who season 4, episode 2, "The Fires of Pompeii," Gillan played a Soothsayer named Mira, marking her Doctor Who debut a full season before she was cast as Amy Pond.

Doctor Who also isn't beyond casting actors as the Doctor who have previously played other characters within the franchise. For instance, Twelfth Doctor Peter Capaldi played three roles in total, with the Doctor being his third and final. As well as appearing as Lobus Caecilius in "The Fires of Pompeii," Capaldi also played John Frobisher in Torchwood season 3 - a Doctor Who spinoff created by Russell T Davies.

Doctor Who Season 15's New Companion Could Be Mundy Flynn From "Boom"

Mundy could travel with the Fifteenth Doctor next season

Varada Sethu could be unique among previous companion actors in that she may have already played her biggest character. In other words, it could be that Sethu's character in "Boom" is who she is set to play in Doctor Who season 15. If Mundy Flynn is the Doctor's companion after Millie Gibson leaves the show, then the dynamic would certainly be interesting. The Doctor and Mundy meet in incredibly dire circumstances, with the Doctor's temper flaring and his nerves close to the surface.

If Sethu isn't reprising her role as Mundy Flynn in Doctor Who season 15, then this implies she is playing an entirely original character.

There aren't many companions who first meet the Doctor at his lowest ebb, and it would make for a compelling foundation on which the pair could build a friendship. In addition, as a soldier, Mundy would have a very different approach to finding solutions to various problems. The Doctor is predominantly a peaceful character who always tries to avoid conflict when necessary, so this mismatched pairing would be exciting to explore.

If Sethu isn't reprising her role as Mundy Flynn in Doctor Who season 15, then this implies she is playing an entirely original character. However, given the cerebral nature of Doctor Who's Disney era with its concealed clues, subtle winks, and fourth-wall breaks, it seems unlikely that the show is replicating its past issues of repeat castings. That being said, it does remain a possibility that Sethu impressed Davies with her time on set and simply earned herself a larger role.

Is Varada Sethu's Early Doctor Who Debut Part Of A Larger Mystery?

Doctor Who season 14 has already teased the presence of unanswered questions