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Karen Gillan and Arthur Darvill, who play companions Amy Pond and Rory Williams on Doctor Who, will be leaving the series after the upcoming season, head writer and executive producer Steven Moffat announced Thursday.

"The Ponds will be rejoining us next series. But the final days of the Ponds are coming," Moffat said at a news conference for Doctor Who's upcoming Christmas special, according to the UK Mirror. "I am not telling you when and I won't say how. But that story is going to come to a heartbreaking end." As depressing as this news is to fans, it should be noted that Moffat refers to both characters as if they share Amy's last name, not Rory's.

The Doctor is a lonely Time Lord though, so the companion void will be filled soon enough. Moffat teased,  "And then the Doctor is going to meet a new friend and I am not going to tell you about he, she or it." The Doctor Who Twitter account, however, tweeted that more details will be available Friday.

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Gillan, 24, joined the series in 2010 as the first companion of the Eleventh Doctor, portrayed by Matt Smith. Darvill, 28, began as Amy's fiancé Rory, who eventually became the Doctor's companion and married Amy. "We have had the most incredible journey and we had to hold hands at first when we joined," Smith said. "It is very disappointing, but this show is about change. It is always about looking forward."

BBC, which produces and distributes the long-running sci-fi series, told the Mirror it was a "mutual decision" for Gillan to exit. She succeeded previous companions such as Catherine Tate and Billie Piper.

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In an interview with The Los Angeles Times posted Wednesday, Gillan — who is currently appearing in a play on London's West End — said she hoped to pursue more projects on the stage in the future. "I want to do more theater. I want to do more stage. I have a real passion for it. I want to do it as much as screen stuff, in all honesty, so that would be really amazing. Maybe something on Broadway. I'd love to do that. Not a musical, maybe like a play on Broadway."

As for that third companion River Song, whose complicated backstory has been the source of much drama this past season, Moffat commented that she will probably be returning in some capacity.

Earlier this year, Smith hinted at this own departure from the series, but he is signed on to appear in at least 14 more episodes, which would premiere sometime in 2012 and run through 2013.

The Doctor Who Christmas special airs Saturday, Dec. 24 at 8/7c on BBC America.