Basically working backwards(pre regeneration) but going forward with Amy for 13 episodes
I only had the roughest idea. Had David stayed for one final year, it would certainly have been his last, so my pitch was that it would start with the TARDIS crashing in Amelia's back garden - as now - and a terribly battered and bruised Tenth Doctor staggering out. Amelia finds him, feeds him fish custard (no that was for Matt, it would have been something more Davidy) and generally helps him. But we, the audience, can see he's in a truly bad way. Dying maybe. Eventually he heads back to his TARDIS, and flies off.
But when he returns - many years later for Amy - he seems perfectly fine, and indeed doesn't remember any of those events...And of course over time, we realise what we saw was the Tenth Doctor at the end of his life, about to regenerate.
Events that we return to in Episode 13...
Interestingly Moffat's version of things where the doctor is nearing his end(Big Bang or proposed season 5) all involve seeing Amelia.
I wonder if he'll do the same when Smith steps down.
We get a glimpse of Caitlin(little Amy) who's a few years older the last time we saw her