Doctor Who does Western (for the first time in a long time) and it's pretty good. Not quite hitting the heights, but a lot better than last week. In essence it's a morality tale, like many Westerns I suppose. Seeking to define the good guys and bad guys with some surprising twists.
This is a dense story that defies a quick summary. Read the Doctor Who Wiki article for the outline. There are so many themes: morality, justice, good vs evil, redemption all interwoven.