The exciting climax of ‘The Day of The Doctor’ sees the Doctor recruiting his past and future incarnations in an effort to save Gallifrey. This continues the theme throughout the episode of planting a seed in the Doctor’s past so that it can reach its conclusion in the 11th Doctor’s present.
This time it is performing the calculations need to preserve the entire planet, which begins with the 1st Doctor. Not only are they doing the calculations but all 13th TARDISes are shown flying around the planet.
Such a gathering must be an extreme rare occurrence, if not unique. The laws of time would normally prohibit this but it must be more than that as since it is possible why did the Time Lords, in the midst of the Time War, not do something similar since it could potentially increase their forces by a factor of 12?
The obvious answer is that bringing your past incarnations into the conflict is a bad idea, since if they die you’ll die as well. It would also be disheartening for a Time Lord to try and recruit his future incarnations only to find he has none (since he is going to die in the war).
It could be that the Doctor is the first Time Lord to think of doing this. It could be such a social taboo, as dangerous, that no one has ever contemplated passing information back along their own timeline or tried recruiting their other selves.
The Doctors conversation in the Tower of London indicates that having the same Time Lord in close proximity runs the risk of a nasty paradox. From ‘The Angels Take Manhattan’ we know that a paradox can be physically destructive to the surrounding area.
Having each Doctor in their respective TARDISes could reduce that potential for paradox. While the TARDIS has shown a reluctance to be near itself the distances shown here could be far enough apart to allow them to co-operate.
‘The Five Doctors’ show an example of how trying to have the same Time Lord in the same place can lead to destabilisation. If not for the efforts of Gallifrey the Doctor could have faded away completely.
The fact that the Doctor’s time stream is already in flux helps makes this possible, maybe in conjunction with the Moment. The 7th Doctor mentions crossing the boundaries that divide one universe from another so they might be slightly out of phases with each other.
In any case once the Doctor’s return to their proper place in their own time stream in all likelihood they would forget what had transpired. This is important to preserve their own personal history.
This still leaves how they were recruited by the other Doctors. This is something that can be used as a framework for a series of linked adventures featuring all the previous Doctors. Like the comic ‘Prisoners of Time’ each adventure could be a different Doctor and their own adventure, ending with them being contacted. Instead you might focus on the War Doctor, 10th and 11th Doctor seeking out their earlier versions.
Just how would the previous Doctors react to their future selves asking them to do something so dangerous? How would they feel about saving Gallifrey?
The 1st and 2nd Doctor were on the run from their own people. What could their future incarnations say to convince them to now help them? How would the 9th Doctor react to learning that there was a chance to undo the destruction of his world, since that guilt still burnt within him?
There is also a question of at what point in their personal timelines they are recruited. Since the 3rd Doctor is able to fly his TARDIS this must be at some point after ‘The Three Doctors’.
Someone must have given the 1st Doctor some guidance as he was never able to fly the TARDIS with such a degree of finesse (and the 2nd Doctor struggled on occasion). It could be that by linking minds they improved their piloting skills or that one of the other Doctors is remotely flying them.
The fact that the 7th Doctor changes back and fore between clips from the television series and the tv movie is either something we should overlook as the viewer or evidence that the Doctors were experience a collapse of the time differential.
The lack of companions in the images shown could indicate either that they are in another part of the TARDIS or they were left somewhere safe (after all, no need to drag them into the Time War as well). Still, it is a shame that the 4th Doctor didn’t have Romana aiding him. Her skills with the TARDIS could have come in very handy.
It is possible that the various Doctors met beforehand to discuss their plan. This could be the misty world that Clara finds herself in at the end of ‘The Name of The Doctor’. The 11th Doctor might just have mistaken them for ghosts, when in fact this was an event in his future and the previous versions were just trying to get away before he arrived and learnt too much about what was in store for him.
We don’t see the end of this assembly of Doctors either. The War Doctor, 10th and 11th Doctor are allowed their goodbyes and hints at the future. It could be that before this scene the other Doctors were present, each leaving one by one.
What would the Doctors ask about their future and what would they tell their past selves? What would they want to say to say to earlier incarnations now they have the chance? Which incarnation is each Doctor’s favourite?
You could use this rare occurrence to fit in more multi-Doctor adventures. It could be that they were dealing with something very important when they were called away and in return for helping their future selves they might want them to return the favour.
There could also be complications with each Doctor returning to their time streams. Different incarnations might end up going on an adventure together before they return home or one or more could simply go missing, forcing the others to go looking for them.
While this all centres on the Doctor and should not be an option for other Time Lord PCs you might allow them one very special occasion to do something similar. This is best done when you at least have an idea about what form each of their incarnations take.