The one constant in the world of Doctor Who is change. It's been embedded in the very fabric of the show's DNA since the very beginning. People usually cite the process of regeneration to prove this point, the means by which the various actors who have played the Doctor have swapped in and out of the role since Patrick Troughton first replaced William Hartnell in 1966. But the same has also always been true of the Doctor's companions, who've been popping in and out of the TARDIS virtually since the series started. Some of the Doctor's friends have stayed for multiple seasons, and some just a handful of episodes, but each has been irrevocably changed in some way by their adventures with the infamous Time Lord. But that doesn't make the process of saying goodbye any easier, particularly when it's someone we feel like we've only barely gotten the chance to know.
Such will apparently be the case for Millie Gibson, who is leaving the show after a single season. Gibson's Ruby Sunday, the first companion of Ncuti Gatwa's Fifteenth Doctor, was introduced in the 2023 Christmas special, "The Church on Ruby Road," and will star in the series' upcoming fourteenth season, which is set to premiere in May. But Variety has confirmed that she won't be part of the show's currently filming Season 15, and has been replaced by Andor actress Varada Sethu.
No other details about Sethu's role have been revealed and neither the BBC, Bad Wolf, or showrunner Russell T. Davies have commented on the casting reports as yet. (But the report has not been denied and Gatwa himself posted an Instagram story that felt like a comment on the larger casting discourse.)
To be fair, a companion leaving after a season isn't all that uncommon. Though Billie Piper starred in two seasons of the show during Davies's first turn as showrunner, subsequent companions Freema Agyeman and Catherine Tate both only stuck around for a season each. Later companions like Pearl Mackie and John Bishop also only had limited time in the TARDIS. And, of course, companions dipped in and out of Classic Who adventures all the time. (Hartnell's Doctor had something like ten companions over his three-year turn in the role.)
Part of the reason this all feels so jarring is likely because we've still only seen Gibson in a single episode, so it feels especially unfair that Ruby --- who is honestly a fantastic character with so much potential --- is being taken away from us so quickly. But because Doctor Who is currently filming quite far in advance, it's also easy to forget that we're still going to get a whole season of adventures with Ruby before she departs.
Season 14 will feature everything from an episode set in Regency England to a visit to the Beatles' famous 1960s recording studio, Abbey Road. It will involve guest turns from performers as varied as Jinx Monsoon, Jonathan Groff, Indira Varma, and Lenny Rush, while featuring the return of classic companion Bonnie Langford and Yasmin Finney as Donna Noble's daughter, Rose. In short, there's a lot to look forward to, so perhaps we can all save our worrying about the future until we've at least seen what the rest of this year has in store.
After all, as the Doctor himself once said, “Everything ends and it's always sad, but everything begins again, too. And that's always happy. Be happy.”
Doctor Who will return in May on BBC One and BBC IPlayer in the U.K., and Disney+ everywhere else.