Doctor Who: Spearhead from Space: Episode 3

The Doctorgets a Stetson (and a gun!), and finds himself the reluctant Sheriff of a Western town under siege by a relentless cyborg, 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.