Well, the title really does sum up the story pretty well. We start out in the 19th century wild west where the US cavalry are doing their best to realize their “manifest destiny”. Which, of course, includes fighting with the Native Americans who naturally aren’t too keen on the violent white invaders. So far, pretty accurate. And then a spaceship crashes. And its inhabitants view humans as primitive (and highly disposable) life forms. Irony! Now it’s up to the humans to put aside their quarrels in the name of preserving the human race. A bit obvious, but a lot of fun.