‘Time Crash’, by Steven Moffat, is a 2007 mini episode for Children in Need. Due to the 10th Doctor failing to raise the TARDIS shields he collides with the 5th Doctor’s TARDIS. The resulting paradox threatens to blow a whole in the universe until the 10th Doctor saves the day by carrying out the actions he remembers watching himself perform when he was the 5th Doctor.
This brief adventure sets the ground work for ‘The Day Of The Doctor’, particularly in how an earlier incarnation finds the 10th Doctor irritating and childish (in this case the 5th Doctor calls him a skinny idiot). The solution also is resolved by the Doctor passing along information to his younger incarnation.
We learn that unless the TARDIS has its shields active it can collide with other time ships (also the Titanic). If this happens they can merge, displaying occupants. Here the 5th Doctor finds himself in the 10th Doctor’s console room.
We’ve seen that the TARDIS can retain multiple console rooms so it is conceivable that the 5th Doctor’s version was just displaced elsewhere within the ship. It is likely that his travelling companions (whoever they may be as he doesn’t confirm who he is with) are also displaced and there could be an adventure to be had in what they got up to during this merging.
Given just how big space and time is the odds of actually colliding with another time ship, let alone the same TARDIS, would appear to be astronomical. Was the 5th Doctor passing through the same region? It is interesting that the 10th Doctor only pulls a lever before the collision occurs. There isn’t even the few seconds it takes for the TARDIS to dematerialise before disaster strikes.
It is therefore tempting to think that there could be another force at play here to engineer these unlikely events. Since the 5th Doctor is involved the Master or the Black Guardian would make good candidates.
The universe is endangered by the two time zones at war at the heart of the TARDIS, creating a paradox. The implication is that this is because it is the same time ship but it could be that all TARDIS have their own internal time zone which is incompatible with others if merged.
This makes a certain amount of sense as a TARDIS is outside of time and we’ve seen examples of the ship protecting occupants from changes to history. This could allow Time Lords to survive even if the universe is rewritten.
The Doctor states that this will cause a paradox but this usually on applies if one event contradicts another. This wouldn’t seem to apply if a TARDIS just finds itself alongside a future version of itself, since this doesn’t change the fact that both will exist. It could be that since its future and its past are both ‘now’ it will destabilise the TARDIS.
The technobabble solution involves a black hole and supernova at the exact same instant, somehow cancelling the explosion and implosion out. It isn’t clear if the 10th Doctor piloted the TARDIS into an actually black hole and supernova at the same time or whether he created these conditions within the TARDIS (as the 5th Doctor does warn him his actions will cause the TARDIS to explode).
Whatever he did do the 5th Doctor realises that even he isn’t smart enough to come up with that solution. The 10th Doctor confirms that he only knew what to do from remember these events.
Like ‘Space/Time’ the answer creates itself through time travel. As discussed in my article about that story this could just be an elimination of possible futures. If the Doctors weren’t able to save their TARDIS then the time ship wouldn’t exist for the 5th Doctor to collide with. The timeline would momentarily reset to allow it to exist and for the collision to occur until they finally find the right combination. Both versions of the Doctor would be unaware that they could have played out these events billions of times.
The 10th Doctor does state that the 5th Doctor will remember this. This suggests the amnesia of crossing ones own time stream won’t apply here. He is surprised when the 5th Doctor appears but it is easy to see how he might loose track of just when this meeting will happen.
He very quickly seems to recover from his surprise and mentions the potential crisis before the alarm sounds. While the 5th Doctor is panicking he knows exactly how things will turn out.
It is possible to interpret this as his memories of the encounter unlocking once the TARDIS merges. Certainly he has no qualms revealing aspects of the 5th Doctors future, including mentioning that the Master will have a wife.