The Doctor gets a Stetson (and a gun!), and finds himself the reluctant Sheriff of a Western town under siege by a relentless cyborg killing machine, who goes by the name of the Gunslinger. But who is he and what does he want? The answer seems to lie with the mysterious Kahler-Jex, an alien doctor (yes, another one) whose initial appearance is hiding a dark secret. Matt Smith stars, with Karen Gillan and Arthur Darvill.