When the announcement was made, Davies said: "They're back! And it looks impossible – first, we announce a new Doctor, and then an old Doctor, along with the wonderful Donna, what on earth is happening?
"Maybe this is a missing story. Or a parallel world. Or a dream, or a trick, or a flashback. The only thing I can confirm is that it’s going to be spectacular, as two of our greatest stars reunite for the battle of a lifetime."
The announcement said the pair are filming scenes for the anniversary, leaving the size of their contribution in doubt and the possibility open that their involvement could just be a cameo appearance.
However, the assertion that they will "reunite for the battle of a lifetime" suggests that this will not be a fleeting appearance, and they will be fully engaged in the action.
Perhaps their return will therefore play out similarly to Tennant's previous appearance in 50th anniversary special The Day of The Doctor, where he had a substantial role alongside Matt Smith's Eleventh Doctor. If so, will more Doctors return for a full-blown multi-Doctor special, as fans have been clamouring for? Only time will tell.
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As for Davies' teasing of how the pair will reunite, we certainly have our theories, ranging from the return of the meta-crisis Doctor to Tennant actually playing the Fourteenth incarnation of the Timelord, rather than reprising his role as the Tenth. Check out our Doctor Who theories for more.
Doctor Who is available to stream on BBC iPlayer. For more, visit our dedicated Sci-Fi page or our full TV Guide.
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