Doctor Who seems to have wrapped up Jack Harkness and River Song's storylines, but Steven Moffat concedes there is always a slight chance that the two companions could return.

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Throughout the Tenth Doctor's travels, viewers saw the death of Captain Jack (John Barrowman) as the Face of Boe and the demise of River (Alex Kingston) in the Library. Although the fates of both characters have long been revealed, the wibbly wobbly nature of time means either character could ultimately find their way back into the plot.

"It's never, ever closed off. It's all about 'Do you have a story that's worth it?'" Moffat explained to Zap2It. "Unless you've got a story that is absolutely informed and invigorated by their presence, just bringing a character back in an ordinary story won't do."

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It is for this reason that any return from either of the beloved characters would need to be part of a major storyline.

"Both River Song and Jack Harkness are massive characters in 'Doctor Who' mythology," Moffat said. "Their returns would have to be seismic, not routine."

Catch Doctor Who when it returns to the BBC and BBC America on Dec. 25 for the annual Christmas Special. Click the video below to see a preview for "The Last Christmas."

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