The Doctor becomes sheriff in a Western town.
The Gunslinger is looking for The Doctor. But Doctor Who?
Doctor Jex created a bunch of RoboCops to eradicate the enemies of their planet's war.
The Dark Doctor is back.
Amy talks The Doctor off the ledge.
The other doctor decides to end the pain he has caused by self destructing.
Instead of allowing the gunslinger to do the same, The Doctor suggests he protect Mercy.