The new season of Doctor Who is in full swing, and it’s already making some big moves early into its run. This week’s episode, “Boom,” puts the Doctor and Ruby on the planet of Kastarion 3, which is currently in the midst of a war the pair are unable to escape from after the Doctor is trapped by an active landmine. That doesn’t stop them from making friends amidst the conflict, though—in fact, one of the allies they gain made for quite a pleasant surprise.

Anglican Marine Mundy Flynn, who helps the Doctor and Ruby throughout the episode, is played by Andor’s Varada Sethu, who’s set to be the new (and currently unnamed) companion in the next season. It appears Sethu won’t reprise the role when she comes over full-time, something made clear on Twitter when she talked about taking her first steps into the franchise through Mundy. “I’m so grateful I get to come back for more,” she wrote, “but you’ll all have to wait and see how that manifests in the new companion role.”

Talking to the Hollywood Reporter, episode writer Steven Moffat wasequally tight-lipped on how her double casting came about, and deferred to Sethu’s appaearance here as all part of showrunner Russell T. Davies’ plans. However, he did note there was some fun in doing a classic Who casting move of hiring guest actors who end up playing companions down the line. In the modern Who era, it happened to both Freema Agyeman and Jenna Coleman—the former’s guest role was later made into the cousin to eventual her companion role of Martha Jones—and even Peter Capaldi landed a guest part before he ended up becoming the Twelfth Doctor.

Davies reaffirmed Sethu’s statement that Mundy’s story has come to an end, and that her companion character will be revealed in full next year. Whether you believe him or not, her showing up helped make the episode even more exciting, and it makes the wait for her to hang with the Doctor and Ruby feel even longer.

Let us know what you thought of “Boom” in the comments below, and look for our recap of the episode on Monday.

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