An English baron, Sir Roger de Tourneville, is mustering a force to aid King Edward III in his wars in France.
And then a Wersgor spaceship arrives over the small town in Lincolnshire where Roger is mustering. The little blue men inside the scoutship attack, the English storm the ship and kill all but one of the aliens, and Sir Roger gets a bright idea: force the surviving alien, Branithar, to fly the English force to France, and from there they can fly to the Holy Land. Sounds great, right?
Unfortunately for them, Branithar has other ideas, and plots a course for Tharixan, a Wersgor colony world, and thus, Poul Anderson's 1960 novel The High Crusade begins. Originally serialized in Astounding/Analog Science Fiction and Fact, it was published as a novel by Doubleday that same year.
Like Three Hearts and Three Lions, the story is told within a framing device. In in, an expedition from Earth comes across a curious historical record of how a bunch of Medieval Englishmen became the first humans to travel the spaceways. The main narrative is told by Brother Parvus, a scholar and clergyman accompanying Sir Roger's wild adventure.
The Englishmen, being medieval, are flabbergasted by the alien technology they encounter, but Roger counters with a deep cunning born in a feudal court system, and a tireless bravado that borders on recklessness. Outnumbered and outgunned by the hostile Wersgorix, his greatest advantage is his boldness in seizing the initiative and outmaneuvering the rigid “advancements” of the aliens. They are so used to impersonal, long-range warfare that when the English draw them into close quarters, the outcome is invariably the same.
The goal is to return home to Earth, but as the hope of that dwindles, Sir Roger's drive pushes him to conquer Tharixan, which strains his relationship with his wife, Catherine. She seeks succor in the company of the handsome young Sir Owain Montbelle, and you can guess that there are going to be problems.
The High Crusade is commonly billed as a satire, and in many ways it is. The idea that a bunch of Medieval Englishmen can be ripped away from Earth and go about conquering a mighty space empire seems silly. It is silly, when described that way.
And yet, the strength of the novel is that it plays everything completely straight. There is no single trace of irony, nor a smirk at the audience that this is as absurd as it sounds. It is committed to the kayfabe and that's how the story is able to work.
Because the story is told with a straight face, it absorbs the reader into it, and it transforms from a “ha ha, Deus Vult in space” concept into a swashbuckling adventure story about a nobleman forced to rise to the position of a conqueror. The climax of the story isn't the conquest of the Wersgorix Empire, but is rather about resolving the crumbling marriage of Roger and Catherine. With sword fighting.
I can't recommend The High Crusade enough. Its an absolute joy to read, both as an action-comedy and as well-done speculative fiction. Anderson wrote a sequel called Quest in 1983, and there was a movie adaptation in 1994 by Roland Emmerich, which is apparently terrible.