In the middle of "The Dying Detective"...
The Doctor is alone in the TARDIS, trapped in the Cardiff Rift, unable to help his Companions and stop Professor Moriarty's machinations. So before he is Driven to Suicide, he withdraws into himself and calls upon someone as a last resort to help him: his own human self, John Smith.
John agrees to help only because he can tell the Doctor is on his very last leg.
This is the last time the Doctor will appear in the series until "Together Or Not At All."
